Describing functions for nonlinear optical systems

The concept of describing functions is useful for analyzing and designing nonlinear systems. A proposal for using the idea of describing functions for studying the behavior of a nonlinear optical processing system is given. The describing function can be used in the same way that a coherent transfer function or optical transfer function is used to characterize linear, shift-invariant optical processors. Two coherent optical systems for measuring the magnitude of the describing function of nonlinear optical processors are suggested.     

Analytical solution to the optical transfer function of a scattering medium with large particles

A new analytical expression for the optical transfer function of multiple-scattering media such as clouds, mists, and dust aerosols is given in terms of their microphysical characteristics. The geometrical optics approximation is used to find local optical parameters of a scattering medium, including the simple approximation of the phase function, which is the key to the solution of the problem considered here. The optical transfer function is taken within a small-angle approximation of the radiative transfer theory. A comparison with Monte Carlo data shows a fairly satisfactory accuracy of our analytic formulas.     

Deriving the Coltman Correction for Transforming the Bar Transfer Function to the Optical Transfer Function (or the Contrast Transfer Function to the Modulation Transfer Function)

The Coltman series for obtaining the optical transfer function from measurements of the bar transfer function is mathematically derived. The bar transfer function rather than the contrast transfer function is defined so that the relation remains valid when an image exhibits phase reversal.     

Analysis of the performance of a wireless optical multi-input to multi-output communication system

We investigate robust optical wireless communication in a highly scattering propagation medium using multielement optical detector arrays. The communication setup consists of synchronized multiple transmitters that send information to a receiver array and an atmospheric propagation channel. The mathematical model that best describes this scenario is multi-input to multi-output communication through stochastic slow changing channels. In this model, signals from m transmitters are received by n receiver-detectors. The channel transfer function matrix is G, and its size is n x m. G(i,j) is the transfer function from transmitter i to detector j, and m > or = n. We adopt a quasi-stationary approach in which the channel time variation has a negligible effect on communication performance over a burst. The G matrix is calculated on the basis of the optical transfer function of the atmospheric channel (composed of aerosol and turbulence elements) and the receiver's optics. In this work we derive a performance model using environmental data, such as documented turbulence and aerosol models and noise statistics. We also present the results of simulations conducted for the proposed detection algorithm.     

Electrically controlled integrated optical filter

The possibility of electric control over the spectral transfer function of an integrated optical filter comprising a reflection Bragg grating in a photorefractive waveguide is demonstrated for the first time. The filter has a spectral selectivity of 0.15 nm, ensures continuous tuning within ～0.1 nm, and can be electrically switched on and off. The proposed method of control is effective and has good prospects for practical applications.     

2R optical regenerator in As2Se3 chalcogenide fiber characterized by a frequency-resolved optical gating analysis

We present a detailed analysis of a 2R optical regenerator based on self-phase modulation in As(2)Se(3) chalcogenide glass fiber using frequency-resolved optical gating (FROG). We obtain good agreement between the FROG measurements and theory, and confirm that the output pulses are near-transform limited. We show that two-photon absorption improves the profile of the power transfer function while not degrading the temporal performance.     

The role of microcrystalline structure on optical scattering characteristics of semi-crystalline thermoplastics and the accuracy of numerical input for IR-heating modeling

Infrared (IR) heating is widely used for thermoforming of thermoplastic polymers. The key benefit of radiation heating is that a significant amount of the radiative energy penetrates into the polymers thanks to their semi-transparency. For the case of heating unfilled semi-crystalline polymers, the relation between their microcrystalline structure and optical properties is the key to develop a predictive IR-heating model as microcrystalline structure introduces an optically heterogeneous medium. In this study, a relation between the microcrystalline structure of a polyethylene (PE) and its effect on the thermo-optical properties was experimentally analyzed considering a two-step analysis. At very first step, the relation was analyzed considering samples with identical thicknesses and different morphologies, characterized here in terms of degree of crystallinity (X_c (%)). Using Fourier Transform Infrared (FT-IR) spectroscopy and integrating sphere, optical characteristics of the PE samples were analyzed in near-infrared (NIR) and middle-infrared (MIR) spectral ranges. The analyses showed that a slight variation in X_c (%) has a great effect on the optical characteristics of PE, particularly the transmission characteristics in NIR range. The wavelength-dependent effect of X_c (%) on the transmission behaviors opened a discussion about the fact that the microcrystalline structures -in particular spherulites or their substructures such as lamellae- are responsible for optical scattering. Using the optical properties obtained from the two-step experimental analyses, two different thermo-optical properties were calculated, namely extinction and absorption coefficients, and used as a numerical input for the parametric numerical studies. The numerical studies were performed using an in-house developed radiation heat transfer algorithm -RAYHEAT-. Both the experimental and numerical analyses demonstrated the importance of the optical scattering regarding the identification of thermo-optical properties, used as a numerical input for radiation heat transfer models.     

In-band noise suppression using novel monolithically integrated semiconductor optical amplifier-based ultra-compact interferometers

Novel all-optical noise suppressors based on the nonlinear transfer function properties of monolithically integrated active waveguide interferometers are proposed and demonstrated. Through a power map imbalance between the two arms of the interferometers, each of which contains multi-contact semiconductor optical amplifiers, a nonlinear transfer function is created, which can then be exploited to achieve in-band noise suppression. The authors demonstrate the use of such a mechanism in ultra-compact Mach-Zehnder and Michelson interferometers (MIs). Experimental work demonstrates a 5.0-dB optical signal-to-noise ratio improvement for the Mach-Zehnder and an 8.4-dB improvement for the MIs, respectively. It is shown that for input data the Mach-Zehnder is capable of providing a Q factor improvement of 4.1 dB. To the authors knowledge, these devices constitute the smallest integrated interferometer structures reported to date demonstrating in-band noise suppression.     

Adjustable active optical low-pass filter

This paper presents an active optical low-pass filter (AOLPF) capable of changing the resolution of an imaging system on demand in order to remove aliasing noise from a sampling image. This is advantageous over conventional optical low-pass filters, which are fixed image-blurring optical components that are built into the imaging system, in order to remove aliasing in the image. Furthermore, conventional filters smear images regardless of the presence or lack of high spatial frequency, which can exceed the Nyquist limit of the sensor. On the contrary, the proposed AOLPF can dynamically adjust the modulation transfer function of an imaging system to eliminate aliasing artifacts. In addition, this filter can be turned off in the absence of high spatial frequency to maximize resolution and prevent unnecessary blurring of the sampling image.     

Two-dimensional binary halftoned optical intensity channels [optical wireless communications]

The capacity of two-dimensional (2D) optical intensity channels in which transmit images are constrained to be binary-level has been considered. Examples of such links exist in holographic storage, page-oriented memories, optical interconnects, 2D barcodes as well as multiple-input/multiple-output wireless optical links. Data are transmitted by sending a series of time-varying binary-level optical intensity images from transmitter to receiver. Neither strict spatial alignment between transmitter and receiver nor independence among the spatial channels is required. The approach combines spatial discrete multitone modulation developed for spatially frequency selective channels with halftoning to produce a binary-level output image. Data are modulated in spatial frequency domain as dictated by a water pouring spectrum over the optical transfer function as well as channel and quantisation noise. A binary-level output image is produced by exploiting the excess spatial bandwidth available at the transmitter to shape quantisation noise out of band. A general mathematical framework has been presented, in which such systems can be analysed and designed. In a pixelated wireless optical channel application, halftoning achieves 99.8% of the capacity of an equivalent unconstrained continuous amplitude channel using lmegapixel arrays.     

Deconvolution-based spatial resolution in optical diffusion tomography

The role that deconvolution plays in the achievable spatial resolution in optical diffusion tomography is examined for the case of imaging an inhomogeneity in an otherwise homogeneous medium. It is shown that, in the measured data, it is the shape of the turbid medium modulation transfer function that determines the maximum spatial resolution. When the turbid medium transfer function is deconvolved from the measured data, it is the signal-to-noise ratio properties of the Fourier-transformed measured data that determine the maximum spatial resolution. It is shown that deconvolution-based methods can improve the spatial resolution in measured data up to a factor of 5 for realistic noise levels. These results are demonstrated with computer-simulated data.     

Amplified double-coupler double-ring optical resonators with negative optical gain

Optical resonators with a double-coupler and double-ring configuration incorporated into optical amplifiers that have negative gain are analyzed. The resonators are presented with a unique signal-flow graph together with z-transform variables for sampled optical signals. Their optical transfer functions are obtained by a graphical technique. The poles and zeroes in the z plane of the transfer functions are examined, which leads to some unique design features of the resonators for optical-filtering applications.     

Visible-band testbed projector with a replicated diffractive optical element

Raytheon has designed, fabricated, and tested a diffractive-optical-element-based (DOE-based) testbed projector for direct and indirect visual optical applications. By use of a low-cost replicated DOE surface from Rochester Photonics Corporation for color correction the projector optics bettered the modular transfer function of an equivalent commercial camera lens. The testbed demonstrates that a practical DOE-based optical system is suitable for both visual applications (e.g., head-mounted displays) and visual projection (e.g., tactical sensors). The need for and the proper application of DOE's in visual optical systems, the nature and the performance of the projector optical design, and test results are described.     

光学图样的仿真处理

主要阐述了利用计算机模拟再现各类彩色光学图样的问题。提出了RGB三原色与可见光光谱的转换方法，建立了RGB三原色与单色光波长的函数关系。通过光强度与RGB三原色的结合来控制绘图函数的颜色参数，获得单色的明暗彩色图样并对图样进行了仿真测量及误差分析。     

Selective optical detection of aromatic vapors

A sensitive layer system of amorphous Teflon AF on silver has been coated on a glass substrate. With a monochromatic light source the reflectivity of the layer system as a function of the angle of incidence exhibits the surface-plasmon resonance as well as a set of leaky-mode resonances. These optical resonance phenomena are sensitive to small refractive-index changes that may be induced by diffusion of particles into the Teflon AF layer. On the basis of this effect, the aromatic vapors benzene; toluene; and o-, p-, and m-xylene have been investigated with different vapor concentrations. By selection of a distinct angle at a particular resonance, dynamic measurements can be performed. Assuming a diffusion process in accordance with Fick's law, the diffusion profile can be calculated as a function of time. As described by the Lorentz-Lorenz relation a refractive-index profile is induced that consequently interacts with the electromagnetic fields of the optical modes. With the function of the diffusion-induced refractive-index profile the shift of the resonance lines can be calculated from the measured reflectivity change as a function of time. The characteristic diffusion coefficients of the particular vapor allow for a distinction between the different types of aromate, even between the different xylenes.     

Correction of optical phase aberrations using binary-amplitude modulation

We show that phase aberrations in an imaging system can be mitigated using binary-amplitude masks that reduce destructive interference in the image spatial frequency domain. Appropriately designed masks increase the magnitude of the optical transfer function and prevent nulls. This offers a low-cost, transmission-mode alternative to phase correction as used in active and adaptive optics, without a restriction on the waveband of operation.     

Quality parameters analysis of optical imaging systems with enhanced focal depth using the Wigner distribution function

An analysis of the Strehl ratio and the optical transfer function as imaging quality parameters of optical elements with enhanced focal length is carried out by employing the Wigner distribution function. To this end, we use four different pupil functions: a full circular aperture, a hyper-Gaussian aperture, a quartic phase plate, and a logarithmic phase mask. A comparison is performed between the quality parameters and test images formed by these pupil functions at different defocus distances.     

双光楔高精度角度发生器设计

从折射定理出发,给出旋转双光楔的入射光线和出射光线之间的对应关系。据此提出可旋转双光楔角度发生器系统的设计方案,该方案利用光楔对光线的偏折来实现对一定视场任意角位置光线的获得。设计了能够提供满足10°圆视场的高精度角度发生系统。     

A low cost high resolution optical position sensor

A fast high-resolution analog position sensor that provides real-time analog output corresponding to the position of the centroid of a diffuse optical source focused on its surface, is described. Theoretical performance equations are presented. Linear and two-dimensional position sensors have been fabricated and tested to verify the basic theory. Analytic results for system static transfer function and transient response are presented and verified experimentally for a 3-μm p-well CMOS process. The main component sensitivities are identified and verified using computer-aided modeling techniques