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Design optimization and experimental studies of bandpass optical filters useful in the near-IR region from 1200 to 2000 nm are carried out. It is seen that the use of Si/SiO2multilayer coating in the form of a multicavity Fabry-Pe´rot (F-P) filter, either alone or in combination with a long-wave-pass (LWP) filter, on an intrinsic silicon (Si) substrate simplifies the filter configuration, obviating the use of the conventional cemented filter configuration. The filter manufacturing studies demonstrate the production of filters with high in-band optical transmittance T>0.90and excellent rejection T<0.001from 400 to 2000 nm. The spectral stability of the filters with respect to temperature is found to be sensitive to substrate temperature; the higher the substrate temperature, the higher is the spectral stability. For filters deposited at a substrate temperature of 600 K, the spectral stability is in the range of 2.27 to 1.58×104K,over a filter temperature from 298 to 373 K, which is mainly dictated by the refractive index coefficient (RICFT) of Si films. © 1999 Society of Photo-Optical Instrumentation Engineers.

A new method for design of multilayer antireflection coatings (ARCs) which makes use of a specified reflectance value R_0, either zero or nonzero at a particular wavelength λ_0, is developed. It offers an explicit scheme for evaluation of the optical/geometrical thickness of each layer, and the design variables are optimized with respect to integrated reflection loss R* over a specified bandwidth. It has been found that the method can be applied for design of wideband ARCs useful in the visible and IR regions, and the resulting bandwidth and residual reflection losses can be controlled as a function of R_0. The method is found to be comprehensive so that more useful designs can be worked out, and it can be directly adopted to the design of single- and double-layer ARCs.

A new algorithm for the design of multilayer antireflection coatings (ARC) is developed. This makes use of a closed-loop nonlinear approximation technique to arrive at the exact thickness of each layer. The method offers a simpler and faster optimization procedure. It has been successfully used in the design of broadband ARCs on substrates with a wide range of refractive indices. Detailed analysis of the effect of errors in the refractive index and thickness of each layer, individually and collectively on the ultimate performance of the system, is discussed in detail. To minimize the effects of these errors, in situ correction (dynamic correction) procedure at each stage of development of the system is suggested.

A detailed study of a new method of determining the optical properties of absorbing materials is presented. It makes use of normal incidence reflectances from the specimen itself (Rs) and from the specimen coated with a transparent film of two different thicknesses but of the same refractive index (R_1s and R_2s) in the form of R_1s/Rs and R_2s/Rs. It is seen that a simple goniometer can be easily adopted for measuring the reflectance ratios over a wide spectral range. The versatility of the method has been proved by the fact that it has been successfully adopted for specimens with surface structures varying from atomically smooth to rough surfaces.

A generalized theoretical formulation for multilayer antireflection coatings (ARCs) has been established, which has scope for considering both zero and nonzero reflectance at one or more than one wavelength. Starting with this, a new design method for three-layer ARCs making use of input reflectance parameters at two wavelengths (R_1 and R_2 at λ_1 and λ_2) has been developed. It allows for determination of all the unknown design parameters explicitly without any a priori conditions on them. The scope of the method has been extended by incorporating optimization of the resultant solutions through an optimizing parameter, integrated reflection loss R* to obtain efficient designs with respect to spectral bandwidth, residual reflection loss, etc. The resulting hybrid methodology is found to be generalized in that it can be used to design a wide range of ARC systems, broad band to V-type, irrespective of the nature of the substrate (absorbing or nonabsorbing) and spectral region of use, with adaptability to simpler/complex ARC systems and other thin film systems such as dichroics/beam splitters and achromatic reflectors.

A generalized reflectance method for determination of optical properties of absorbing materials is developed and compared with other reflectance methods. In the present scheme the specimen is coated with dielectric transparent layer(s) and the reflectance ratios are measured. This novel scheme of specimen preparation and the method of measurement allow the specimen to be free from surface layers and at the same time account for possible effects of surface roughness. It can be applied to a wide variety of materials regardless of their surface conditions and is particularly useful for metals.