Study on Longwave IR Filter for Remote Sensing Instrument

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 | n₀ and
(2) n_s| LH2LHLHLH2LHLHL | n₀.

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 ~ 14mm. 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.

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

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