New advances in improving low-temperature stability of infrared thin-film interference filters

Abstract

The degeneration of performance of an optical thin-film interference filter associated with the change of temperature is not acceptable. In this letter, we report a new progress in improving low-temperature performance of infrared narrow-band filters by using Pb 1-x Ge x Te initial bulk alloy with appropriate Ge concentration x. It can be found that there exists a critical temperature for the investigated narrow-band filter, at which the temperature coefficient of filter is exactly zero. Therefore, by means of controlling the composition in (Pb 1-x Ge x ) 1-y Te y layers, the temperature coefficient of filter can be tunable at the designated low-temperature. In our present investigation, when temperature varies from 300 to 85 K, a shift of peak wavelength of 0.05935 nm.K -1 has been achieved. INTRUDUCTIONThe performance of an optical thin-film interference filter can be influenced by the change of ambient temperature.The peak or cut-off wavelength will shift and the pass-band shape and peak transmission will deteriorate due to the effects of varying temperatures. In particular, when the filter is used in light-wave communication systems or spaceborne remote sensing instruments, the degeneration of performance is not acceptable. It becomes difficult to sustain the functional performance of non-dispersive narrow-band interference filters for precision spectroradiometric measurements from space 1 . It is not practical to add an auxiliary temperature control to a narrow-band filter in order to maintain its stable optical performance in spaceborne remote sensing systems.There are two factors that cause the instability of the performance of a thin-film interference filter accompanied by the change of temperature: one is the temperature-induced variations in the indices of refraction of layers; another is the variations in the physical thicknesses of layers. Since the bulk temperature coefficients of linear expansion are an order of magnitude smaller than the temperature coefficients of refractive index for substances employed for interference filters, it may be speculated that the shift of wavelength should be ascribed to the variations of temperature coefficients of indices of layers 2 .By classical design, optical interference filters typically consist of multiple homogeneous layers of two materials with low and high refractive indices. As far as the infrared interference filters employed in the spaceborne remote sensing systems are concerned, as a rule, the materials are lead telluride (PbTe) for high-index layers, and either zinc

Infrared High-Index Coating Materials, PbTe and Pb1−xGexTe: Properties and Applications

“…When the ambient temperature varies, the performance of an optical thin-film interference filter will be also changed, such as the shift of center wavelength and the deterioration of peak transmission [34][35][36]. In particular, when a long wavelength infrared narrow bandpass filter is used in the spaceborne remote sensing instruments, the change of its spectral characteristics, which will lead to the difficulty to sustain the precision radiometric measurements from space, is not acceptable [37].…”