Characteristics Of Optical Filters Research Articles

Distributed Bragg reflector active optical filters

The characteristics of active optical filters based on distributed Bragg reflector (DBR) laser diodes are described. The authors present a theoretical model for the small and large signal filter response which can be used to optimize the filter design and operating point. The large signal filter response is dominated by a variety of nonlinear effects, including a wavelength domain bistability. The authors describe a series of experiments in which the predicted filter characteristics are demonstrated. The authors present detailed measurements of the filter transfer function in both transmission and reflection modes under low and high power injection. For the small signal measurements a powerful technique which uses the swept frequency concept is presented. For the large signal regime, measurements of both frequency pulling and wavelength domain bistability are presented. >

Accurate infrared transmittance measurements on optical filters using an FT-IR spectrometer

The accurate measurement of transmittance characteristics of optical filters in the IR region of the spectrum is a demanding application for an IR spectrometer. We have demonstrated that the well known advantages of the Fourier transform IR spectrometer (FT-IR) can be utilized for this application. Measurements using FT-IR may be made faster and with better SNR than with dispersive instruments. In addition, the measurements have an optical precision that is superior in wavelength accuracy and equal to, or better, in transmittance accuracy. Instrumental parameters and test procedures necessary to obtain reliable and reproducible data are discussed in detail. The spectral characteristics of filters in both the near-and mid-IR regions are presented, covering a total range from 1-50 microm (10,000-200 wavenumbers). The types of optical filters examined included those with narrow and wide bandpass, and shortpass characteristics.

Comprehensive moment generating function characterisation of optically preamplified receivers

An accurate and soundly based representation of the complex stochastic processes encountered with optical preamplification in direct detection receivers is obtained. The objective of this study is the identification of a comprehensive moment generating function (MGF) characterisation for practical optically preamplified receivers. To effect this we build on a skeleton abstract formulation outlined some time ago with only limited physical and structural interpretation by Personick. Here we develop a MGF for the postdetection noise processes which takes into account a comprehensive set of key amplifier and receiver parameters, including gain and spontaneous noise characteristics of the semiconductor laser amplifier (SLA), amplifier facet reflectivity, bandwidth of the associated optical filter and postdetection electrical receiver filter characteristics. This comprehensive MGF may be used to obtain, via an independent route, the simple mean and variance characterisation of Yamamoto. We demonstrate this correspondence on a term-by-term basis and identify also minor discrepancies in two terms of the Yamamoto noise representation. The importance of the new MGF formulation is that it provides, for the first time, a sound formal basis which may be adopted for receiver design and analysis; this will enable, in any given situation, the adequacy or otherwise of a simple Gaussian approximation to be assessed.

Characteristics of coupled-mode optical filters

We provide a quantitative picture of the characteristics of a CdS coupled-mode band-pass optical filter. The operating principle of these coupled mode filters (CMF) is the energy exchange between the ordinary and extraordinary modes when the sample, inserted between two crossed polarizers ∥ and ⊥C, is strained by uniaxial stress at 45° from the C axis. The filter operates at the isotropic wavelength λi where ne=n0 ensures phase matching. The transmission rate and the selectivity are investigated as functions of the stress magnitude T and the sample thickness e. The best results are obtained by adjusting T so that the coherence length Lc=2e. For a 1-mm-thick crystal, the half-height passband is ∼4 Å, while the transmission rate is ∼70% at 77 °K and 20% at 300 °K. The large field of view, in practice ∼45°, constitutes a significant advantage over interference filters. We compare the CMF with zero crossing birefringent filters (ZCBF). Around λi, the characteristics of both filter types are rather similar, but the single transmission window of CMF gives the latter a distinct advantage over ZCBF. Transmission control by variation of applied stress is an added advantage of CMF.

Configuration dependence of optical filtering characteristics in backward nearly degenerate four-wave mixing

Backward nearly degenerate four-wave mixing close to the atomic resonance line has been investigated experimentally. It has been found that phase mismatch provides a window factor to the reflectivity that masks the frequency spectrum of the medium and that the line profile depends on the geometrical configuration of the three interacting beams. The capability of obtaining an optical bandpass filter with nearly degenerate four-wave mixing has also been demonstrated experimentally.

Christiansen effect in IR spectra of soil-derived atmospheric dusts

The Christiansen effect occurs when a suspension of particles in a transparent medium is observed at a wavelength where the refractive indices of the particles and of the medium are equal, thus producing an optically homogeneous medium with optical bandpass or filter characteristics. It is shown that soil-derived atmospheric dusts and other particulates can exhibit the Christiansen effect at IR wavelengths and that their spectra can be simulated by equivalent, precipitated films of the particulates on optical substrates. Useful approximation equations are given. The implications of these observations should be considered by atmospheric physicists and IR systems designers.

Multilayer Dielectric Stacks of Coupled Filters

The variety of optical filter characteristics obtainable with periodic stacks of two dielectric materials having simply commensurate optical thicknesses can be greatly increased when the basic section consists of more than one pair of layers. Although no more difficult to monitor in coating than stacks of two-layer sections, coupled-filter stacks have stopband spectra which differ considerably from those of their components in much the same way as the transmission characteristics of closely coupled resonators differ from those of their component resonators. The utility of periodic coupled-to-idler frequency ratios not filled by simply periodic multilayers; and two which permit the construction by band-nesting of wide band high-reflectance mirrors for near-ir and visible wavelengths, as well as MacNeille-Banning prisms for the uv region, with lower index ratios than previously envisaged.

Temperature compensation of birefringent optical filters

Temperature characteristics of birefringent optical filters were improved by two orders of magnitude by combining calcite and LiTaO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> crystals in series. Theoretical treatment of the temperature compensation and experimental results are presented.

A mechanical version of the Smith Chart

A modification of the Smith Chart has been devised for the purpose of computing the transmission characteristics of non-absorbing optical or electrical multi-element filters. The conventional printed chart is replaced by a system of mechanical linkages together with a spiral computing element. The computer is more accurate, faster and less laborious to use than the orthodox chart.

A Spectrophotometer for Measurement of Optical Filters

Some months ago the RCA License Laboratory, in connection with television development, had occasion to make measurements on the spectral characteristics of optical filters. The conventional and elaborate equipment usually used for such purposes was not immediately at hand, and a cathode-ray oscilloscope coupled to a few simple appurtenances was pressed into service and gave highly satisfactory results. The equipment is easily duplicated and possesses sufficient accuracy for many uses. Design considerations and constructional details are given.