Infrared focal plane detector Dewar components integrate optical elements.
Optical Dewar packaging: Built-in optical components (filters, field lenses, aperture stops, cold shields, etc.) are included.
1. Optical lenses replace planar optical windows (simplifying system design, applicable to focal planes).
2. Cold shield integrated optical lenses (spatial non-uniformity correction?).
3. Variable cold aperture (dual F-numbers F/3, F/6 for four fields of view).
4. Cold aperture integrated planar filters (background limitation, narrowband filtering, multispectral detection).
5. Cold aperture (background limitation, suppression of stray radiation).
6. Focal plane array integrated micro-lenses (circular, square micro-lenses, immersion lenses, light cones?).
Design of cold shields for detectors based on optical system stray light suppression.
Cold shield design for background-limited detectors based on detector performance.
Cold shield, aperture stop integrated as one called cold screen.
Cold screen
Efficiency
Detector quantum efficiency
Different applications for forward-looking infrared, search tracking, and guidance have different requirements.
For munitions, rapid cooling is required, and the cold screen affects cooling time.
Imaging requires uniform illumination across the focal plane, and cold screens cause spatial non-uniformity.
The third generation sensor should be considered as a sensor system, not just as an infrared focal plane array.
The third generation sensor requires precise non-uniformity correction. Spatial non-uniformity must be reduced to less than half the noise equivalent temperature difference. At f/1.8, it must achieve 50mK or less. Spatial non-uniformity should be less than 0.5 noise equivalent temperature differences. Noise equivalent temperature difference at f/2 should be less than 1mK (long wave), and less than 5mK (mid wave). Pixel 1818 (window/micro-lens).
To simplify system design, some degree of non-uniformity correction may be performed on the focal plane along with an optical Dewar data link.
Standard second-generation MCT devices with fine optical coatings and/or closely adhered filters can achieve narrowband filtering, enabling multispectral/hyperspectral arrays.
The area obtainable from the readout circuit is relatively small; micro-lens arrays are needed to restore the duty cycle.
Infrared detectors achieving background-limited (BLLP) performance can reduce the background radiation flux received by the photosensitive element by adding a cold screen on the chip, thereby improving the device’s peak detection rate.
80元HgCdTe linear array device’s cold screen effect and background-limited performance, Gong Haimei, Li Yanjin, Hu Xiaoning, Xu Guosen, Fang Jiaxiong, Journal of Infrared and Millimeter Waves, Vol. 16, No. 3, June 1997: 169-173.
Using a back-illuminated chip with a micro-hole cold screen is an effective means to enhance device performance. The micro-hole cold screen acts as a special cold screen layer on the back of the chip; at the same time, the micro-hole cold screen ensures that the response area of the photosensitive element remains within the design size range, avoiding the issue of expanded response area.Research on suppressing background flux in HgCdTe photovoltaic detectors, Wang Chenfei, Chen Honglei, Li Yanjin, Laser and Infrared, Vol. 37, September 2007: 935-937.
A new opaque detector housing is developed to keep the detector optically and thermally isolated from its surroundings. A box-in-box type housing is cooled to 60K (350F) to reduce the glow or black body radiation emitted by warmer objects surrounding the detector and to prevent additional noise.Israel’s RIT company leads the development of the next generation of infrared detectors, Infrared, Vol. 34 (2013), No. 3: 47-48.
The high sensitivity and high performance requirements of infrared seekers necessitate the optical system to have the following characteristics:
1) 100% cold aperture efficiency. The cold aperture is set to limit unnecessary thermal interference from outside the detector’s field of view, and the design of infrared optical systems for cooled detectors must consider the matching of the exit pupil with the cold aperture to ensure 100% matching efficiency. To achieve this goal, two common methods are currently used: one is to use the cold aperture directly as the optical system’s aperture stop, suitable for short focal length optical systems; the other is to use secondary imaging, suitable for long focal length optical systems, but this method reduces system transmittance due to the addition of steering mirrors and field lenses. Luo Haibo, Shi Zelin, Current Status and Prospects of Infrared Imaging Guidance Technology, Infrared and Laser Engineering, Vol. 38 (2009), No. 4: 565-573.
4) No thermalization. The temperature range of the infrared seeker working environment varies significantly, and changes in temperature affect the curvature, thickness, and spacing of optical elements, as well as the refractive index of the base material and surrounding medium. Most infrared lens materials have a significant change in refractive index with temperature, leading to changes in the image plane of infrared optical lenses and thus affecting image quality. Therefore, it is necessary to design infrared imaging systems without thermalization to compensate for image plane drift caused by temperature changes. Luo Haibo, Shi Zelin, Current Status and Prospects of Infrared Imaging Guidance Technology, Infrared and Laser Engineering, Vol. 38 (2009), No. 4: 565-573.
6) Short and stable back working distance. The length of the back working distance affects the size of the entire imaging system! Its stability affects the stability of image quality. Therefore, infrared seekers require the optical system to have a short back working distance while being insensitive to environmental conditions, under the premise of meeting structural requirements. Luo Haibo, Shi Zelin, Current Status and Prospects of Infrared Imaging Guidance Technology, Infrared and Laser Engineering, Vol. 38 (2009), No. 4: 565-573.
In the field of infrared optical lenses, binary optics and micro-optics technology are noteworthy research directions. Utilizing binary optics and super-resolution technology can simplify the optical system structure, reduce the weight of the optical system, and improve image quality, achieving a large field of view while ensuring high image quality. Micro-lens technology can reduce the light receiving area of the detector, increase the fill factor, improve detection rates, enhance uniformity, and reduce noise.
Luo Haibo, Shi Zelin, Current Status and Prospects of Infrared Imaging Guidance Technology, Infrared and Laser Engineering, Vol. 38 (2009), No. 4: 565-573.
Another direction for the development of infrared imaging seeker technology is integration. Infrared imaging seekers require compact FPA structures, small volume, light weight, and high integration. Driven by supporting technologies such as Dewar and cooling for infrared detectors, the integration of infrared focal plane arrays includes integrating “system on chip (SOC)” on infrared focal plane detector chip components, as well as integrating Dewar with coolers, signal processing electronics components, and optical elements into “packaging systems (SIP)”. The integration of optical elements with detector components allows for more functions, such as wavelength scanning and polarization imaging. Munitions-based infrared focal plane detectors are typically cooled types, consisting of detector chips (including readout circuits), Dewar, coolers, and built-in optical elements (filters, field lenses, and cold shields). The built-in optical elements ensure that the photosensitive chip can receive as much effective infrared radiation as possible, achieving maximum detection rates and optimal performance of the detector.
According to the urgent need for target tracking, reconnaissance, and surveillance at night in modern warfare, infrared optical systems have gained widespread application in the defense field. Compared to zoom infrared optical systems, variable F-number infrared optical systems based on variable cold apertures can switch between large field searches and very small field monitoring, improving the utilization of the aperture and enhancing image quality. As the demand for field range, image quality, and system miniaturization continues to rise, variable F-number infrared optical systems are gradually highlighting their advantages. Variable F-number infrared optical systems can select appropriate F-numbers based on battlefield situations, especially the distance to the observed target, flexibly switching between large field searches and very small field identification and tracking, meeting the reconnaissance and surveillance needs for military targets, with broad application prospects. The third generation forward-looking infrared detector system designed by the U.S. military is a typical application of variable F-number optical systems, characterized by an optical system with four fields of view, dual F-numbers, and a variable cold aperture. The technical routes for achieving variable cold apertures include:
1) Modifying the infrared detector to place the variable cold aperture mechanism inside the infrared detector, achieving an internal variable cold aperture. This internal variable cold aperture is challenging to implement, requiring consideration of cooling, detector modification packaging, and other aspects, but it has the highest integration and is the future direction of variable cold aperture development;
2) Packaging the infrared detector (or focal plane), variable cold aperture, and part of the optical system in one Dewar, sealing and cooling the Dewar to achieve variable cold aperture adjustment. For example, the Dewar device designed in the foreign HALO can add a variable cold aperture in front of the focal plane to achieve the design of the detector’s variable F-number.
3) Through optical design, placing the cold aperture of the infrared detector in front, achieving variable cold aperture design by locally cooling the cold aperture in the optical path. This technical route increases the difficulty of optical system design, and the overall system’s volume and power consumption also increase to some extent, but it does not involve modifications to the infrared detector part, making it highly feasible.
In response to the requirements for controlling the field radiation uniformity of third-generation large-area focal plane arrays, as well as the urgent need to improve optical quality and reduce the volume of optical systems for large-area focal plane array imaging, the optical imaging system lens part is integrated with the Dewar optical system design, making it a compact organic whole.
Typically, the lens part of infrared optical imaging systems is installed outside the infrared detector Dewar component. The optical lens is affected by environmental temperature, which can impact imaging quality. This is because the optical lens installed outside the infrared detector Dewar component is highly sensitive to temperature changes, and the refractive index of the optical lens for infrared radiation varies with temperature. The passive change in refractive index alters the focal position of the optical system, necessitating focal length adjustments to obtain clear images, requiring the introduction of specialized focal length control devices. Additionally, the optical system (including lenses and their mechanical devices) generates infrared radiation like any other object, and the noise introduced reduces the signal-to-noise ratio of the infrared detector. The noise caused by temperature changes in the optical lens can be partially compensated through non-uniformity correction of the output signal, but when the system performs non-uniformity correction, the infrared system cannot image.
Integrating a low-temperature optical system within the Dewar can reduce the thermal noise introduced by the optical system itself, improve the dynamic range and signal-to-noise ratio of the detector, and prevent focal length drift of the imaging system due to environmental temperature changes. During the operation of the infrared imaging system, there is no longer a need to introduce specialized focal length adjustment mechanisms, effectively reducing the frequency of non-uniformity corrections and enhancing the system’s ability to withstand environmental temperature changes. Integrating optical lens components and variable F-number cold screen structures within the Dewar, as well as designing auxiliary structures compatible with the system on the housing, can reduce the overall volume of the infrared detector. This allows for the creation of very compact and lightweight systems, which are highly competitive for many applications.France’s Sofradir company has developed 60° and 180° mid-wave detectors, as well as infrared dual-color fisheye (180°) devices suitable for MWS applications. Israel’s SCD company has developed a 384×480 infrared focal plane detector cooling component that achieves a large field of view of 105°×135.5° without increasing component volume by integrating built-in optical lenses. AIM’s 1280×1024 mid-wave component fully utilizes the space in the housing.
The integrated low-temperature optical system, along with the Dewar cold screen and low-temperature filters, forms a complete structure. In such an integrated optical system, it typically contains 2 to 4 optical lenses, and filters can also be part of the integrated optical system, which can reduce the weight increase of the Dewar cold head introduced by the integrated optical system, aiming to minimize the thermal mass of the entire Dewar cold head. Compared to traditional optical systems, the integrated optical system significantly reduces lens sizes and eliminates focal length adjustment mechanisms, improving imaging quality while also reducing costs.
References omitted.
Li Jianlin 20120518