Single aperture multispectral systems are becoming prevalent thanks to advances in multispectral detectors, new optical materials, and new methods for selecting materials that minimize chromatic and thermal focal shift. This design study focuses on design of a three field-of-view, multispectral lens operating across the MWIR and LWIR spectral regions. The lens in question will have an f-number of f/3 with a 3X zoom ratio. The narrow full field-ofview of the lens is 3.33° with a wide full field-of view of 9.99°, the length of the system is 163 mm. The performance goal for the lens is diffraction limited over the thermal region. The study will provide an overview of material selection using an updated γv-v diagram, to provide achromatic and athermal characteristics. The study will then step through first order layout, optimization with key constraints, and tolerancing for manufacturability. Finally, the study will provide detailed analysis of system performance including as-built MTF over temperature, aberration analysis, and NETD contributions from narcissus.
With the move to more and more lightweight and cost-effective design, a move to multiband or multi-spectral optics is required. These systems are becoming more prevalent in the market as new detector technologies have been developed. However, the lens designs are only starting to be considered with the addition of new materials in the MWIR and the LWIR. For the VIS/SWIR region the designs have been possible, but a lack of detector technology has resulted in few designs being considered for actual manufacturing. These designs are also difficult due to changes in the Abbe number in the different wavebands. Where the glass map is robust in the visible region, there exists a lack of crown glasses in the SWIR, and one is left with mostly flint glasses. This proves challenging from a chromatic perspective. The challenge becomes even more difficult if one wants to incorporate athermalization.
Recently, optical materials have been developed by Schott and NRL to improve material selection in the SWIR, MWIR, and LWIR wavelength regions. In addition, new multiband detectors are reaching maturity, leading to a natural push for common aperture lens systems. Detectors that can span the SWIR/MWIR, MWIR/LWIR or SWIR/MWIR/LWIR wavelengths regions will require complex optical systems to effectively utilize their full potential. Designing common aperture wide-band systems that are both achromatized and passively athermal, especially while maintaining SWAP-c (size, weight, power and cost), poses significant challenges. Through use of the updated γν-ν diagram, which provides guidance on material combinations that both achromatize and athermalize, part of that challenge is reduced. This updated γν-ν diagram uses instantaneous Abbe number and peak wavelength. The instantaneous Abbe number is a function of wavelength and is the scaled reciprocal of the instantaneous dispersion. The instantaneous Abbe number is defined at the peak wavelength, which occurs when the second derivative of the index of refraction goes to zero. Three examples will be presented using this updated athermal/achromatic glass map to demonstrate its effectiveness. These design examples will include a SWIR/MWIR design, a MWIR/LWIR design and, a SWIR/MWIR/LWIR design.
Over the past few years, new detector technologies have enabled multiband detection through a single aperture. This creates significant SWAP advantages (size, weight and power) and has spurred significant interest in multiband optics (for instance SWIR/MWIR, MWIR/LWIR, etc.). However, due to the small number of materials available in the infrared regions, passive optical athermalization and achromatization can be challenging even over single waveband. This becomes even more challenging in the case of multiband optics. One method for determining appropriate material combinations for athermalization and achromatization is use of a <i>y</i> ∗ <i>v</i> vs. <i>v</i> diagram. We examine an updated form of the <i>y</i> ∗ <i>v</i> vs. <i>v</i> diagram using instantaneous Abbe number. While Abbe number is an effective metric for dispersion within single bands, it becomes less reliable when extended to wider wavelength ranges. Instantaneous Abbe number allows for a wider waveband to be defined, without a loss of generality; and this allows for an updated definition of the <i>y</i> ∗ <i>v</i> vs. <i>v</i> diagram for the development of multiband optics. We present an example of a multiband lens as well as compare the typical definition of Abbe number with instantaneous Abbe number to determine the validity of the updated model.
With the move to smaller pixel sizes in the longwave IR region there has been a push for shorter focal length lenses that
are smaller, cheaper and lighter and that resolve lower spatial frequencies. As a result lenses must have better correction
for both chromatic and monochromatic aberrations. This leads to the increased use of aspheres and diffractive optical
elements (kinoforms). With recent developments in the molding of chalcogenide materials these aspheres and kinoforms
are more cost effective to manufacture. Without kinoforms the axial color can be on the order of 15 μm which degrades
the performance of the lens at the Nyquist frequency. The kinoforms are now on smaller elements and are correcting
chromatic aberration which is on the order of the design wavelength. This leads to kinoform structures that do not
require large phase changes and therefore have 1.5 to just over 2 zones. The question becomes how many zones are
required to correct small amounts of chromatic aberration in the system and are they functioning as predicted by the lens
design software? We investigate both the design performance and the as-built performance of two designs that
incorporate kinoforms for the correction of axial chromatic aberration.
A method for performing optical beam shaping in the near-field region using diffractive optical elements generated by
Fresnel based Phased Optimised General Error Diffusion algorithm (POGED) was developed and investigated by means
of numerical simulations. POGED was found to deliver significantly higher signal to noise ratio than iterative
Gerchberg-Saxton type algorithm.
The transposition of signals in space is fundamental to the optical interconnection of electronic systems. Previous free-space implementations employed a scheme based on imaging systems that are prone to aberrations. The system proposed here is useful for implementation of a three-stage Clos network, an optical transpose sector switch, a reconfigurable optical transpose system, and an optical cross connect switch. The purpose of this paper is to propose a non-imaging system for an optical transpose interconnect system, where a macrolens is inserted between the two mesolenses arrays, at the centre. The proposed macrolens is a Fourier transform lens system designed to be virtually aberration free.