Use of a dual band FPA necessitates an optical system that is capable of imaging both mid wave infrared (MWIR) and
long wave infrared (LWIR) spectral bands simultaneously. Such optical system can have up to 10 lenses, (20 surfaces
that require antireflection (AR) coatings) which, if 95% transmitting in each band, will result in overall throughput of
just under 60%1. With 99% transmitting in each band, overall throughput would be just over 90%, a relative
improvement of 50%. An earlier paper presented dual band antireflection designs, as well as early fabrication attempts
on plano Ge, ZnSe, ZnS, AMTIR-1, and CaF2 windows2. This paper presents results of prototype coating fabrication on
ZnSe, Ge, and BaF2 lenses that comprise a 7 lens set. The measured performance of the individual elements is used to
model overall system performance. The elements were incorporated into an optical assembly and measured overall
imager performance is analyzed and presented.
A method of antireflection coating the interior and exterior surfaces of a deep concave optic is under development and is
described. The challenges of coating such an optic include obtaining uniform performance, good mechanical and optical
performance across a temperature range of ambient to 1000oC, and the transition to cost effective production. The
coating process utilizes a tuned cylindrical magnetron sputtering source which sits inside the nose cone to coat the inner
surface and a complementary cylindrical sputtering source to coat the outside surface. The flux from the sputtering
source is tuned along the length of the cylinder by stacking an inner core of magnets in such a way as to produce a
spatially variant magnetic field which allows the source distribution to approximate a uniform deposition on the surface
of the optic. A deposition occulting mask provides fine tuning of source uniformity.
Dual band infrared imagers require a similar set of filters as are needed by single band infrared imagers but with the
added requirement of high transmission in the mid and far infrared. The design of discrete layer filters with optimized
dual band transmission is investigated for three types of filters. These are a visible-infrared beamsplitter, a long
wavelength edge filter and a dual bandpass cold filter. These designs illustrate the role that harmonic reflection bands
can play in the design of dual band filters. The visible reflection beamsplitter design does not have harmonics in the
infrared but requires additional layers to reduce reflection at mid and long wavelengths. The long wavelength edge filter
requires suppression of the second and third harmonics while the sensor band pass cold filter can use harmonics to
advantage. Design techniques are discussed and the results of an initial set of fabrication runs are presented to assess the
sensitivity of example designs to manufacturing errors.
Sensor performance for dual band forward looking infrared (FLIR) imagers can be substantially improved by increased simultaneous throughput of both sensor bands in the optical systems. Currently available antireflection coatings (ARs) have optimized performance for either spectral band, but not both on the same optic. Where AR coatings cover the mid and long wave infrared (LWIR) bands, or the entire broad band spectrum from visible to LWIR, performance is not sufficient for future systems. A method of designing and fabricating high performance ARs has been developed. This paper presents a discussion of the trade-off of film thickness and complexity versus transmission performance. Fabrication results for high, medium and low index lens materials are also presented.
Optical interference notch filters shift to shorter wavelengths with increasing angles of incidence. This phenomenon restricts the filter's field of view and limits the practical application of narrow reflection notch filters. The amount of shift is inversely proportional to the effective average index of the composite film. A method of designing narrow notch optical filters with very broad field of view and controllable bandwidth is demonstrated. Because this method produces a filter that is predominantly composed of the high refractive index material, it will shift on angle less than a typical quarter-wave notch filter. Increasing the effective index of the filter also reduces the separation of S and P-polarized light with angle. This paper presents modeled and measured performance for both mid and far-infrared filters developed using this technique. Narrow notch discrete and rugate filter designs are compared.
The selection of thin film materials for use in far infrared filters is limited. While silicon can be used as a high index material in the far infrared, the suitability of low index materials is less understood. In this study, thin film materials with spectral transmission extending from 1 to beyond 30 micrometer are characterized and evaluated for use in Rugate and discrete interference filters. A materials selection matrix was developed, and five materials were selected for characterization. Transmission, reflection and absorption data are presented for AgBr, AgCl, KBr, CsI and CsBr as single material films, and as blends. These materials are characterized for stress, exposure to humidity, and color center formation when exposed to visible light.
The combination of discrete and Rugate filter design techniques can provide unique design advantages. Rugate filter deposition techniques can be applied to discrete, and square wave based designs as easily as they are applied to sine wave index profiles. Just as apodization reduces sidelobes about a Rugate stop-band, apodization reduces sidelobes of a square wave filter. This paper builds on these observations to present designs which superimpose sine and square wave profiles to produce a more efficient use of the design space. Techniques are presented for moving back and forth between discrete and Rugate designs to achieve reduced film thickness, more efficient use of index contrast, and harmonic suppression.
Fabrication of high performance optical filters are typically long deposition runs. Successful deposition requires the monitoring and correction for long term process variability due to source depletion and equipment degradation as well as control of immediate parameters such as deposition rates and substrate temperature. Multiple resolution analysis of insitu sensor data using wavelet transforms provides the means of extracting both short term and long term trends from a common data stream. This paper presents a method for monitoring the fabrication of discrete and Rugate filters using the Haar transform. This technique begins by describing the average of the time series, and then successively resolves the time series into more detailed layers. The lowest resolution coefficients describe optical density growth and absorption. The more detailed layers describe systematic and periodic variations due to the growth of optical thickness. Separate analysis of data for different time regimes provides insight into different physical parameters. Least square fit to the low order terms relate to parameters such as film absorption and growth of optical density. Windowed Fourier transform of periodic structure in the coefficients of the detailed layer provides insight into a number of performance parameters such as the period and amplitude of the optical thickness cycle which is characteristic of Rugate design.
KEYWORDS: Control systems, Raman spectroscopy, Reflectivity, Refractive index, Optical filters, Gradient-index optics, Crystals, Fabrication, Control systems design, Signal detection
The wide range of optical thin film applications utilizing gradient index coatings has prompted the development of advanced optical control techniques. These include ellipsometric and photometric instruments capable of in-situ measurement of optical performance as the optical structure is being deposited. This paper discusses design sensitivity analysis and instrument configuration for development of a control strategy. The ability to measure optical thickness, refractive index and mechanical thickness is a function of several instrument parameters including wavelength, number of wavelengths, angle of incidence, and complexity of measurement surface. The most critical control data in the fabrication of a particular rugate design, and the instrument parameters and techniques employed and how they affect the control strategy is presented in this discussion.
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