Nonlinear Interference Filters Research Articles

Synthesis of neural pulse interference filters for image restoration

The synthesis of nonlinear pulse interference filters in the form of three-layer and polynomial perceptrons has been considered in this study. Neural filters were used for removing the “salt and pepper“-type pulse interference from half-tone images. It was shown that a three-layer perceptron filter in terms of the filtration accuracy is not inferior to its polynomial analogues and its mathematical model is mush simpler than the Volterra model. Neural and polynomial filters ensure a higher level of pulse interference suppression as compared with median filters.

Theory and implementation of an all optical SR flip-flop based on a pair of externally addressed logic elements

Abstract Holding power criteria for operation of an all optical flip-flop based on a pair of externally addressed nonlinear interference filters are given analytically. It is shown that the holding power level of an individual device cannot be less than half of its own critical switching power for the onset of optical bistability if the device is initially optimised for low power operation. The steady-state and dynamic responses of an operating circuit are also demonstrated and discussed theoretically.

Calculation and design for bistable optical devices of nonlinear interference filters

Three mathematical models for calculating and designing bistable optical devices (BODs) of nonlinear interference filters are compared and the necessity of using the accuracy matrix model based on the principle of thin-film optics to study optical bistabilities of nonlinear interference filters is confirmed. The relationships of important parameters, such as δ l ' 0 , δ l t , and δ l 0 with the initial detuning δ, which are very helpful in designing the BODs, are calculated and plotted. The dispersive relations of ZnSe thin-film are introduced into the method to make the theoretical method self-consistent. Finally, experimental results of a BOD of nonlinear interference filter whose construction is calculated using this method are given.

Optical logic elements based on nonlinear interference filters

A modified nonlinear Fabry–Perot resonator with a bleaching nonlinearity, capable of withstanding higher loads and with an enhanced sensitivity to control signals, is considered as a possible optical logic element.

Refractive optical nonlinearities in a thin film ZnSe interference filter

Picosecond angularly-resolved excite-probe measurements have been used to make the first identification of a negative refractive optoelectronic nonlinearity in the ZnSe material deposited in thin-film nonlinear interference filters. A positive thermal refractive nonlinearity becomes dominant within 6 ps. We attribute the ultrafast relaxation of the photoexcited carriers to surface recombination at grain boundaries within the material.

Coupled-mode theory of diffraction-induced transverse effects in nonlinear optical resonators.

We develop a one-dimensional theory of diffraction-induced transverse effects in nonlinear Fabry-P\'erot resonators addressed by finite-width incident beams. This is achieved in the framework of the coupled-mode analysis, which takes full advantage of the fact that nonlinear Fabry-P\'erot devices are resonant. As compared with a recently published theory dealing with the same subject [M. Haelterman, Opt. Commun. 75, 165 (1990); M. Haelterman, G. Vitrant, and R. Reinisch, J. Opt. Soc. Am. B 7, 1309 (1990); and G. Vitrant, M. Haelterman, and R. Reinisch, ibid. 7, 1319 (1990)], where the nonlinearity is introduced in an approximate way, the formalism developed here takes the nonlinearity associated with the optical Kerr effect rigorously into account. This feature has an important consequence: It leads to a theory that can be generalized to the TM case and also to anisotropic nonlinear media. The theory presented here is valid for any value of the angle of incidence. Under normal (or quasinormal) incidence, two counterpropagating modes, having the same absolute value of the wave-vector component parallel to the plane of the mirrors, are resonantly (or nearly resonantly) excited, whereas under oblique incidence, only one of these modes is excited.This allows us to point out that the feedback leading to optical bistability occurs along the direction of propagation of these resonantly excited counterpropagating modes of the nonlinear optical resonator. This explains why optical bistability disappears under oblique incidence. Although developed in the case of nonlinear Fabry-P\'erot resonators, this theory applies to a wide range of nonlinear optical resonators such as nonlinear prism couplers, nonlinear grating couplers, nonlinear interference filters, etc. The theory developed here applies whenever the excited electromagnetic field can be accurately described by a single mode or two counterpropagating modes of the nonlinear device. As an example, numerical results are given in the case of a nonlinear multiple-quantum-well Fabry-P\'erot-type structure of ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/GaAs with ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/AlAs mirrors.

Dispersive optical bistability in stratified structures.

A versatile, plane-wave transfer-matrix formalism is presented, describing the stationary optical response of multilayered structures where any layer may exhibit a Kerr-type nonlinearity of local or diffusive nature. In order to establish the range of validity of this formalism, the results have been compared to exact solutions both for nonlinear interference filters and superlattices. In practical cases, the nonlinear transfer-matrix formalism is always applicable, except for those superlattice structures where the layer thickness is smaller than the material wavelength.

Research on ZnS non-linear thin films and bistable filters

Non-linear ZnS thin films of excellent quality were prepared by doping with other elements, e.g. copper, using special production techniques. The films were prepared as non-linear interference filters (acting as optical gates). The switching energy density of the filters reached 0.0326–0.0652 J cm −2 for a diameter of laser beam focal spot on the filter surface of 50 μm, a wavelength of 5145 Å and on and off times of 50 μs and 120 μs respectively. Unsealed filters, immersed in water for a few minutes, did not show a significant change in optical bistability.

Dispersive optical bistability in a nonlinear interference filter using an intracavity nematic liquid crystal film with hybrid molecular alignment

Optical bistability in a nonlinear Fabry-Perot resonator due to reorientational effects in a thin intra-cavity nematic liquid crystal film with hybrid molecular alignment has been investigated. The influence of the initial resonator-detuning, which can be altered by temperature control, on the dynamics of the system and the dependence of the transmission characteristic of the nonlinear Fabry-Perot cavity on the cavity-finesse are shown. A refined theoretical way of describing liquid crystals in a Fabry-Perot resonator gives good agreement with experimental results.

Spot-size dependence in thermal nonlinear microcavities

We present a theoretical model of a physically pixellated nonlinear interference filter illuminated by a Gaussian laser beam. The plane-wave model figure of merit of a previous paper is redefined as a function of spot size, pixel dimensions, and the thermal conductivities of the pixel and substrate. We present detailed calculations of switching powers, and actual values are obtained for the cases of glass and polyimide pixels on a sapphire substrate. The theory predicts that switching powers of the order of less than 1 mW can be achieved for the polyimide-sapphire combination and 10 mW for the glass-sapphire combination. An upper limit of the order of 107 gate Hz/cm2 in an array of pixels is projected.

Irradiation-induced switch power drift in optically bistable nonlinear interference filters at 514 and 830 nm

Light-induced changes in the switch power of optically bistable ZnSe nonlinear interference filters have been studied experimentally at 514 and 830 nm wavelengths. This drift was found to be mainly caused by an irreversible change in the peak wavelength and transmission of the filter and is associated with high internal operating irradiances. These changes are apparently due to photostructural modifications of the material forming the central spacer layer. The drift was shown to be dependent on the spacer thickness, incident spot size and the deposition technique used to grow the filter structures. The rate of switch power drift was also found to be reduced considerably when using an illuminating wavelength well removed from the band edge of the spacer material, e.g., 830 nm.

Operation Of An All-Optical Exclusive-Or Gate And Set-Reset Flip-Flop Using Nonlinear Interference Filters

The design of an all-optical exclusive-OR gate and a set-reset flip-flop is described, and the operation characteristics of both circuits using nonlinear ZnSe interference filters is reported. While both functions could be demonstrated, long-term stability in excess of 30 s was not obtained for the flip-flop operation.

Analysis of optical bistability through rapid water desorption from hard coatings

The mechanism of optical bistability in TiO2/SiO2 nonlinear interference filters is investigated and found to be due to water sorption in the pores of the material.

Optically bistable nonlinear interference filters for use with near-infrared laser diodes

ZnSe nonlinear interference filters, configured as optothermal bistable etalons with absorbed transmission (BEAT) devices, have been operated as all-optical switches using 830 nm diode laser illumination and fibre-optic coupling. Stable switching at powers down to ∼3 mW have been demonstrated for spot diameters of 10 μm. An observed increase in switch power when using large numerical-aperture illumination is analysed in terms of a reduction in effective cavity finesse.

Switch-power Drift and Pre-damage Effects in Optically Bistable ZnSe Nonlinear Interference Filters

Abstract Drift of the input power levels at which optically-bistable nonlinear interference filters switch between their transmission states has been studied experimentally. In addition to the small initial drift already observed by various groups, we have monitored, and characterised, a slower irreversible drift associated with the high internal operating irradiances. This pre-damage effect appears to be photo-structural in origin. The results show a small increase in refractive index (∼0·35%) and a decrease in the absorption coefficient (∼20%) over periods ranging from a few minutes to approximately one hour, depending on operating conditions. We have shown that the drift rate can be minimized by going to thicker spacer devices and by using molecular beam deposition techniques to grow the multilayer structures in place of conventional thermal evaporation.

Optical Bistable Devices for Memory and Logic

Nonlinear interference filters and liquid crystal cells for low power optical bistability are reviewed. The use of filters for prototype optical circuitry and a proposed cellular-logic-image-processing architecture are described

Arrays of non-linear thin-film microcavities

Non-linear interference filters exhibit bistability and other modes of operation at room temperature and visible wavelengths which make them suitable candidates for digital optical computing. Parallel processing entails arrays of bistable elements with high packing densities. Generation of the former by simple illumination with a large number of beams on a uniform filter is ruled out because of the long-range thermal coupling (cross-talk) between elements, thus preventing independent operation. Calculations are presented which indicate that arrays of microcavities (or material pixellation) point the way forward to achieve the desired packing densities. Steady state results obtained from the solution of the heat equation for a geometry consisting of a cylindrical pixel mounted on an infinite substrate with the interference filter on top of the cylinder are discussed.

Single-pixel bistability with nonlinear interference filters

We present a theoretical model describing a pixellated, optically bistable, interference filter illuminated by a Gaussian beam. Preliminary experiments using ZnSe devices and 514-nm wavelength radiation show good agreement with this model for pixels of area 80×100 μm2 and a depth of 60 μm. Critical powers and cooling rates are presented for different combinations of materials and pixel dimensions.

A model for pixellation of nonlinear interference filters

We present a theoretical model for the effect of material pixellation on the operating characteristics of optical logic elements using nonlinear interference filters. From our calculations we predict pixel packing densities of 250×250 per cm2 (an increase of three orders of magnitude over optical pixellation) and power levels of 100 μW. We define a figure of merit which describes the effects of pixellation of thermo-optic logic elements. We also show numerically the effect on cooling rate.

PIXELLATION OF OPTO-THERMAL BISTABLE DEVICES FOR SWITCH POWER AND CROSSTALK REDUCTION

We report the reduction of switch power in an optically bistable nonlinear interference filter (NLIF) by pixellation. Pixellation was achieved by machining grooves on NLIF using an excimer laser. We also compare the experimental results with a theoretical model.