Study on Longwave IR Filter for Remote Sensing Instrument
Zhu Lingxin, Zhang Lin, Fan Bin
(Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 200083 Shanghai)
Abstract
Longwave infrared filter is
widely employed in remote sensing
instruments. This paper describes our study
on making IR filter including filter design,
monitoring method, stability under low
temperature and miniaturization. The
successful employ of manufactured filters
verifies the practical value of relative
technologies.
Introduction
Infrared optical film device is one of
the key units in remote sensing instrument.
Its function lies in translating or reflecting
the energy in a certain wave range. The
material used to make longwave IR film
device is limited, some narrow forbidden
band semiconductor material with high
refractive index has bigger light absorption
and the domestic infrared film thickness
monitoring system only has lower precision
at present. So the manufacture of longwave
IR filter is rather difficult. Therefore,
relative study and technical development
possess obvious significance.
Design of Filter
Tolerance analysis of coatings
The tolerance analysis of coatings is
very important for the design of filter with
certain specification. A coating, which can
satisfy the performance requirement and
also has larger tolerance, should be finally
selected. Thus the needed precision of
monitoring system can decrease, and then a
filter with the performance very close to
theoretic calculation can be fabricated.
Example: For a filter in a space-borne
instrument, the tolerance analysis is used to
calculate the performance of the following
two coatings,
(1) n
s| LH2LHLHLH2LHLHLH2LH | n
0 and
(2) n
s| LH2LHLHLH2LHLHL | n
0.
Where L expresses a ZnSe layer
having optical thickness of one-fourth
wavelength and its refractive index n
L = 2.4
-i0.001. H expresses a PbTe layer having
optical thickness of one-fourth wavelength
and its refractive index n
H = 5.5 -i0.008.
Tolerance calculation indicates that the end
product rate of coating (2) is 1.4 times of
that of coating (1). But coating (2) has a
worse wave-shape coefficient than coating
(1). Therefore, one could determine the
suitable coating according to practical
needs.
Filter design of longwave pass filter
The longwave pass filter is for
restraining the shortwave interference and
getting the thermal infrared energy in 8 ~
14
mm. Its traditional coating (0.5LH0.5L)
n
or (0.5HL0.5H)
n generally employs the
thinned basic periodic thickness [0.88
(0.5LH0.5L)] to modify the obvious
corrugation in band pass range.
Unfortunately, the coating error of 0.88
(0.5LH0.5L)] usually results in a worse
change of performance of pass band . We
adopted a new method that reduces the
thickness of H layers in the two sides of the
coating to modify the concave peak. It
shows a perfect effect.
Monitoring Method
For the purpose of making qualified
optical film device as close as possible to
theoretical design, monitoring of the film
parameters must be carried out. The
traditional method is measuring and
controlling the optical thickness of film
layers. Now the infrared coating still mainly
employs such a method that the extreme
value of transmittance or reflectance will
appears when the optical thickness reaches
integer times of one-quarter wavelength.
Besides filter design, the tolerance in
monitoring scheme also should be
considered. That is one could design and
select the best scheme by way of tolerance
analysis method. No doubt, this is an
important progress in monitoring
technology. Table 1 shows an example of
monitoring scheme design using tolerance
analysis.
Tab. 1 Qualification rate of filter performance by various monitoring schemes *2
*1: The qualification rate could be 25% by shifting monitoring wavelength to short direction.
*2: Random error is 0.3. Systematic error is 0.5. Two spectral curves are calculated.
| Ge | LH4LHLHLHLH4LH | n0
| l0= 9.71 ±0.23mm
| Dl= 0.23 ±0.028mm
| T =77 % |
| Coating Monitor | Buffer layer
| Monitoring order | Qualified No.
in wavelength position precision | Qualified No.
in bandwidth
| Qualified No.
in transmittance |
Qualification rate.%. |
| Si | L | 2 | 8 |
8 | 9 | 41 |
| Si | 0 | 2 | 1 | 10 | 7 | 0*1 |
| Al2O3 | 0 | 3 | 11 | 11 | 12 | 83 |
| Si | 0 | 3 | 11 | 11 | 12 | 83 |
According to above calculation result
we can determine the suitable monitoring
scheme for selected coating including
coating monitor, buffer layer and monitoring
order. And then the qualification rate for
filter manufacture by using various
monitoring schemes can be obtained.
