Study on Longwave IR Filter for Remote Sensing Instrument
Stability
Remote sensing instruments often
employ cooled detector. The matched filter
also operates under low temperature like 105
K, 85 K etc. The performance stability of
filter in cooled condition must be
considered.
The main film materials for longwave
infrared filter are PbTe and ZnSe. Their
temperature coefficients equal -210
-3
K
-1
and 410
-4K
-1 respectively. They are used as
high/low refractive index materials. Since
the two temperature coefficients have
opposite sign, this will be benefit to the
stability of central wavelength position
when the film is constructed from them.
Besides, if different film structures,
especially with different spacer material, are
used, the variation of filter performance
(central wavelength, transmission and
bandwidth ) under low temperature are also
different . Tab. 2 is the measured result of
different filters in a temperature-variable
spectrum measurement system that consists
of BIO-RAD FTS-40 Fourier Transform
spectrometer (made in USA) and OXFORD
DN1704 temperature-variable Dewar flask.
Tab. 2 Performance variation of filters from 295 K to 85 K
| Waveband |
8.4 ~ 8.9mm | 10.3 ~ 11.3 mm
|
| Film material | ZnSe, PbTe |
ZnS, PbTe |
| Features | Three half-wave, half-wave layer of ZnSe | Three half-wave, half-wave layer of PbTe
|
| Central wavelength | +0.5 % (+0.045 mm) | +1.7 % (+0.19 mm) |
| Bandwidth | -8 % | -10 % |
| Transmittance | +2.5 % | -19 % |
The shift of central wavelength and
bandwidth variation of these filters under
low temperature (See Fig.1) can be
corrected by better filter design and
depositing operation. The transmittance
decreasing can be restrained by a
technology of enhancing the film layer
density.
Fig.1 Spectral curves of 11.5— 12.5 mm bandpass filters
In order to improve the stability of
longwave infrared filter, SITP studied the
PbGeTe material having high refractive
index. The dopant Ge can make the
temperature coefficient of refractive index
of PbTe change from negative to positive.
As for the filter using PbGeTe as high
refractive index material, the shift of central
wavelength tends to zero and the
transmittance basically has no change at low
temperature of 85 K. Its stability is
obviously better than that of the filter using
PbTe.
Miniaturization of Filter
The development of detector array in
remote sensing spectral technology requires
the miniaturization of relevant dispersion
devices. For example, the three-element
HgCdTe detector used in a space-borne
instrument has a center-to-center distance of
0.4mm. It asks the width of filter covering
each detector element also to be 0.4mm.
Through coating the film on a large
substrate with thickness of 0.17mm and
cutting it being a required dimension, the
miniature filter were obtained. The break on
the edge is less than 0.02mm that meets the
usage requirement. As for the filters in
dimension of centimeter order, they can be
formed by the way of rigging up and are
used also in the same way. Then a good
reliability will be obtained. These
miniaturization methods are simple and
feasible.
Conclusion
We have developed some methods and
technologies for making longwave infrared
filter. The manufactured filters possess fine
optical performance and reliability. Many of
them were successfully employed in various
airborne/space-borne remote sensing
instruments. It verifies the practical value of
these technologies.
References
- H.A.Macleod, Monitoring of Optical
Coatings, Applied Optics, 1981, Vol.
20, No.1, pp82~ 89.
- H.A.Macleod, Error compensation
mechanisms in some thin-film
monitoring systems, Optica Acta,
1977, Vol.24, No.9, pp907~ 930.
- Zhu Lingxin, Yan Yixun and others,
PbTe coating material and properties
of its deposition films, Chinese
Journal of Infrared Research, 1985,
Vol. 4, No.1, pp1~ 8.
- A.M.Zheng, J.S.Seeley and others,
Ultranarrow filters with good
performance when tilted and cooled,
Applied Optics, 1992, Vol.31, No.22,
PP4336~ 4338
