Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, L.go E. Fermi 2, 50125 Firenze (Italy) Ph.+39 (055) 23.07.51 DIPARTIMENTO DI ASTRONOMIA E SCIENZA DELLO SPAZIO DEVELOPMENT OF AN INFRARED WIDE PASS BAND GERMANIUM FILTER A. De Sio (a) , A. Giannini (a) , G. Dionisio (a) , L. Gambicorti (a,b) , P. Bianchi (b) , M. Ciofini (b) , L. Mercatelli (b) , E. Pace (a,b) (a) Dep. Of Astronomy and Space Science - Firenze University (Italy) (b) National Institute of Applied Optics – National Research Council, Firenze (Italy) Technical Report TR01-2008 Version 1.0 Date: 03 2008
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Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, L.go E. Fermi 2, 50125 Firenze (Italy) Ph.+39 (055) 23.07.51
DIPARTIMENTO DI ASTRONOMIA
E SCIENZA DELLO SPAZIO
DEVELOPMENT OF AN INFRARED WIDE PASS BAND
GERMANIUM FILTER
A. De Sio (a), A. Giannini(a), G. Dionisio(a), L. Gambicorti(a,b), P. Bianchi
(b), M. Ciofini
(b),
L. Mercatelli(b), E. Pace
(a,b)
(a)Dep. Of Astronomy and Space Science - Firenze University (Italy) (b) National Institute of Applied Optics – National Research Council, Firenze (Italy)
Technical Report TR01-2008
Version 1.0
Date: 03 2008
Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, L.go E. Fermi 2, 50125 Firenze (Italy) Ph.+39 (055) 23.07.51
2. EXPERIMENTAL.................................................................................. 4 2.1. The thin film optical simulator........................................................ 4
2.2. The optical fabrication facilities ...................................................... 4
2.3. The optical treatments deposition facility......................................... 5
2.4. The optical characterization facilities............................................... 5
3. PROCEDURES, RESULTS AND DISCUSSION........................................... 6
3.1. The theory of the anti-reflex monolayer .......................................... 7
3.2. The Optical simulations ................................................................. 8
3.3. The Optical simulations ................................................................. 9
4. CONCLUSIONS: The IR optical window prototype................................. 12
DEVELOPMENT OF AN INFRARED WIDE PASS BAND GERMANIUM FILTER Technical report
Ref.: DASS-1th_Tech_rep_2008.doc 3
ABSTRACT
Many applications, such as IR astronomical sources observations, Earth observation
from space, environmental monitoring or volcano monitoring, require the development
of imaging systems working in the Infra-Red (IR) spectral range. Such systems
require optics and filters able to cut the visible radiation and to maintain high
transparency in the IR spectral range above 2 µm. The design and the fabrication of
an optical window for such type of imaging systems are described in this report. The
optical window will work in the 7 - 14 µm range and has been fabricated on a
Germanium substrate. Ge was selected for its negligible absorption coefficient in the
spectral range of interest. Unfortunately, its transmissivity is limited to 45%, because
of its high refraction index (n = 4.3 @ 10µm). In order to increase the transmissivity
of the window and to avoid ghosts generated by multiple reflections, a ZnS
antireflection coating has been deposited on both the two window surfaces.
The development phases from the optical simulation through the calibration of the
deposition machine and the fabrication of test prototypes to the manufacturing of the
optical window will be here described.
1. INTRODUCTION
Germanium is a very useful material to develop IR imaging systems. Owing to its
physical characteristics, it is an ideal substrate: it is a robust material with a negligible
absorption coefficient in the wavelength range between 8 µm and 14 µm. Above 14
µm the absorption coefficient increases because of some absorption bands.
Unfortunately, Ge has a high refraction index (4.3 at 10 µm) in the interval 2 µm –
14 µm and consequently reflections occur at the air-Germanium interfaces. Such
reflectivity reduces the efficacy of the Ge window, since they cut a 55% of the IR
transmitted radiation intensity and it produces ghosts generated by multiple
reflections inside the material. Furthermore, if images are taken using a Ge window, a
reflected image of the camera on the first Ge surface is often superimposed to the
picture.
DEVELOPMENT OF AN INFRARED WIDE PASS BAND GERMANIUM FILTER Technical report
Ref.: DASS-1th_Tech_rep_2008.doc 4
Therefore, a ZnS monolayer coating was selected in order to reduce the reflectivity
and to increase the transmittance at the two interfaces of the Germanium window.
The choice of ZnS was driven by the well know adhesion between Ge and ZnS and
because its refraction index is close to the square root of the refraction index of the
Ge. In such case the reflections on the surfaces can be nullified.
Aim of this technical report is to describe the design, the experimental facilities and
process of optimization to obtain a ZnS monolayer coating to improve the
transmissivity of a Germanium window dedicated to IR imaging system.
2. EXPERIMENTAL
2.1. The thin film optical simulator
The reflectivity or transmittance can be calculated at different wavelengths and
plotted by using optical simulators and ray-tracing software. Real materials have
optical constants depending on wavelength, so it is fundamental to know their real
values. Realistic calculations of optical properties must include such variations. The
values of the refraction index and the extinction coefficient as a function of
wavelength for each material are stored in a database that is accessible to the
simulation programs so, any dispersion can be effectively modeled. Material behavior
is not always ideal and the optical constants often depend on the particular coating
machine and on the deposition parameters. The operating conditions affect the
material properties, but they can be also simulated.
2.2. The optical fabrication facilities
The optical fabrication was carried out in the laboratory of CNR-INOA (National
Institute of Applied Optics of Firenze), where basic machines are located (two-axes
raw surfaces process, diamond-wire cutting machine, patinas processing, glass cutter,
etc). Other laboratories are equipped with an optical finishing and polishing machine,
a curvature ray measuring machine, etc. In the third laboratory there are an
apparatus that is used to manufacture optical fibers and an interferometer, a
prototype developped at INOA, to measure the material surface quality through the
optical fringe technique.
DEVELOPMENT OF AN INFRARED WIDE PASS BAND GERMANIUM FILTER Technical report
Ref.: DASS-1th_Tech_rep_2008.doc 5
The available instrumentation in each laboratory can be divided into three main
classes:
� Instruments/machineries or the raw material and/or patinas processing (sawing