Angular correlation function of speckle patterns scattered from a one-dimensional rough dielectric film on a glass substrate

We present the experimental results of the angular correlation function of far-field speckle patterns scattered by a one-dimensionally random rough surface of a thin dielectric film on a glass substrate when a polarized beam of light is incident upon the rough surface from vacuum. This surface, which separates the vacuum and the dielectric, is rough enough that only diffused speckles are observed. The experiment for the correlation measurement was set up to make use of a CCD camera to obtain the image of the speckle pattern in the specular direction for each given angle of incidence; the cross-correlation function is then calculated from the digitized images. It is found that the intensity correlation functions exhibit two distinct maxima: one arises from the autocorrelation and the other from the reciprocity condition. It is also found that different scattering processes give rise to quite different correlation functions: multiple-scattering processes produce narrow peaks with secondary maxima and single-scattering processes produce relatively broad peaks.     

Minimum-phase-function-based processing in frequency-domain optical coherence tomography systems

We present a simple processing technique that uses the concept of minimum-phase functions to improve frequency-domain optical coherence tomography systems. Our approach removes the autocorrelation noise and therefore increases both the accessible depth range and the recovery accuracy. To our knowledge, this is the first time that the concept of minimum-phase functions has been applied to improve optical coherence tomography.     

Source coherence impairments in a direct detection direct sequence optical code-division multiple-access system

We demonstrate that direct sequence optical code- division multiple-access (DS-OCDMA) encoders and decoders using sampled fiber Bragg gratings (S-FBGs) behave as multipath interferometers. In that case, chip pulses of the prime sequence codes generated by spreading in time-coherent data pulses can result from multiple reflections in the interferometers that can superimpose within a chip time duration. We show that the autocorrelation function has to be considered as the sum of complex amplitudes of the combined chip as the laser source coherence time is much greater than the integration time of the photodetector. To reduce the sensitivity of the DS-OCDMA system to the coherence time of the laser source, we analyze the use of sparse and nonperiodic quadratic congruence and extended quadratic congruence codes.     

A Comparative Study for the Assessment on Blood Flow Measurement Using Self-Mixing Laser Speckle Interferometer

We study a self-mixing laser diode (SM-LD) as a low-cost compact optical sensor for noninvasive blood flow measurement over the surface of the skin. We compare the SM-LD system with a commercially available Doppler flowmeter to assess the accuracy and feasibility of the SM-LD sensors for such applications. For the SM-LD flowmeter, we apply two different signal processing methods: (1) the counting method, i.e., counting the intensity fluctuations of the signal to obtain a frequency value, and (2) the autocorrelation method, i.e., measuring the autocorrelation time of the signal. In vitro measurements show good agreement with the commercially available flowmeter. In vivo measurements performed on test subjects revealed that the autocorrelation technique shows much better results. The results of in vitro and in vivo studies and the comparison with the commercial flowmeter confirm the applicability of the SM-LD flowmeter.     

Diversity detection of speckles for double-wavelength interferometry on rough surfaces

When the topography of a rough surface is measured with a double-wavelength interferometer, the phase error of the signal corresponding to the synthetic wavelength increases in the vicinity of dark speckles. To overcome this problem we perform an amplitude-dependent averaging of the synthetic phase over independent speckles (diversity detection). We either use spatially neighboring speckles or in the case of depolarizing surfaces, we use speckles of the same spatial mode, but with orthogonal polarizations. For the latter case the lateral resolution stays unaffected. The reduction of the speckle noise is demonstrated experimentally for a laterally scanning double-wavelength interferometer with superheterodyne detection of the synthetic phase.     

Optomechanical integration method for finite integrals

We present, for the first time to our knowledge, an optomechanical integration method for finite functions. This technique allows for the integration of any finite function by combining optical and mechanical principals. The integrated function can then be determined using curve fitting methods. Furthermore, the original function can be reproduced through numerical or analytical integration.     

Autocorrelation and Power Measurement with Pyroelectric and Dielectric Bolometers

A pulse comparison technique is described which yields the autocorrelation function and the power spectrum of a repetitive time-domain waveform. The autocorrelation function is realized with a sliding short in a coaxial transmission line to provide time delay; a pyroelectric bolometer to provide multiplication through a square law voltage response; and a capacitor to provide integration. Problems of realization of a perfect time delay and integration limitations are considered, and it is found that noise fluctuations yield the main time resolution limitation that is equivalent to 8 ps for 15-V pulses and a 50-s integration time. The pyroelectric voltage-sensing bolometer is then compared to a pyroelectric capacitance sensing bolometer. It is shown that the capacitance sensing bolometer can handle much longer pulse durations than the voltage sensing bolometer. It is also demonstrated that the sensitivities of the two techniques are equivalent in a typical case at a capacitance sensing bolometer bridge voltage of 3 V. Measurement results of the autocorrelation function and power spectrum, using a voltage sensing pyroelectric bolometer, are given for a nominal 15-V, 500-ps time duration, whose baseband pulses have a 100-pps (pulses per second) repetition rate.     

Twin-sensitivity measurement by spatial multiplexing of digitally recorded holograms

We present an all-numeric multiplexing/demultiplexing technique for digitally recorded holograms that allow the simultaneous determination of the in-plane and the out-of-plane components of the displacement vector of an object submitted to some loading. The twin-sensitivity measurement is obtained from two different illumination directions that give two sensitivity vectors. The spatial multiplexing is achieved by an incoherent mixing of two duplets of coherent waves that produce holograms carried with orthogonal polarized reference waves. The spatial demultiplexing uses the autocorrelation function of the multiplexed holograms as a position estimator. The estimator then enables the determination of the pixel-to-pixel correspondence between the holograms for the double component determination. Results of the experiment are presented.     

基于FPGA高速信号采集的多角度动态光散射法纳米粒径测量

根据动态光散射装置测量纳米粒径原理,开发了一套基于现场可编程门阵列(FPGA)的纳米粒径测量系统。该系统通过光电倍增管(PMT)输出光子脉冲信号,利用FPGA实现高速脉冲采集及自相关运算,采用双脉冲计数器实现高精度可控的连续计数,并实现DDR3异步存储以及USB通信交互等接口功能。自研板卡既可实现自相关函数实时采集运算,又可无丢失地保存海量原始数据信号。采用该系统对200nm聚苯乙烯颗粒进行了测量,分析了不同采样时间及延迟时间等参数对粒径测量结果的影响。实验结果表明:自研FPGA采集板卡测量重复性为1.2%,具有很好的稳定性和重复性。     

Influence and compensation of autocorrelation terms in depth-resolved spectroscopic Fourier-domain optical coherence tomography

We demonstrate depth-resolved spectral absorption measurements in the wavelength range from 750 to 850 nm using a broadband light source consisting of three spectrally shifted superluminescent light-emitting diode modules and a low-cost spectrometer-based Fourier-domain optical coherence tomography system. We present the theoretical model and experimental verification of interferences between autocorrelation terms and the signal carrying cross-correlation terms, strongly affecting the absorption measurements. A simple background subtraction, minimizing the artifacts caused by the interferences of autocorrelation and cross-correlation terms, is presented.     

Dynamic light scattering with single-mode receivers: partial heterodyning regime

A frequent source of errors in dynamic light-scattering experiments is partial heterodyning caused by scattering on large particles or imperfections of the sample cell. With a conventional two-pinhole receiver it is impossible to distinguish its effect from the effects of a finite detector area and detector nonlinearity. However, an accurate data analysis is feasible when a single-mode light receiver is employed. We present formulas for single-mode autocorrelation and cross-correlation functions that include a local oscillator and an incoherent background of arbitrary strength and take into account detector nonlinearity (e.g., dead time) up to second order. A simple but accurate method for the determination of the nonlinearity parameters and the effective number of receiver modes is also provided. The success of the data-evaluation procedure is demonstrated by the measurement of the hydrodynamic radius of latex in the presence of deliberately added local-oscillator or incoherent-background contributions.     

Some space-time statistics of the turbulent point-spread function

We consider the point-spread function (PSF) of an imaging system looking through weak optical turbulence along a horizontal path through the atmospheric surface layer. From this PSF we derive a scalar total irradiance field and a center-of-mass vector field. Theoretical values are found for the space-time autocorrelation and cross-correlation functions for these fields, which are then compared with the observed correlation functions obtained from data taken at the Validation Measurements on Propagation in the Infrared and Radar (VAMPIRA) measurement trial. We discuss the meaning of these results and possible directions for future work.     

A study of the turbulent characteristics of a free circular jet of an incompressible gas

Results are presented for the measurement of the velocity, temperature, and intensity of variations in these quantities within a free circular jet. Data are presented on the distribution law for these variations and for their derivatives with respect to time, as well as on the autocorrelation and spectral functions of the fluxuations in velocity and temperature.     

Statistics of intensity in adaptive-optics images and their usefulness for detection and photometry of exoplanets

This paper is an introduction to the problem of modeling the probability density function of adaptive-optics speckle. We show that with the modified Rician distribution one cannot describe the statistics of light on axis. A dual solution is proposed: the modified Rician distribution for off-axis speckle and gamma-based distribution for the core of the point spread function. From these two distributions we derive optimal statistical discriminators between real sources and quasi-static speckles. In the second part of the paper the morphological difference between the two probability density functions is used to constrain a one-dimensional, "blind," iterative deconvolution at the position of an exoplanet. Separation of the probability density functions of signal and speckle yields accurate differential photometry in our simulations of the SPHERE planet finder instrument.     

Absorbance measurements in turbid media by the photon correlation method

Optical absorption in highly turbid media was quantified by the time shift of the electric field autocorrelation function of diffused photons. The intensity autocorrelation function was analyzed by the third-order cumulant expansion and a linear relationship between the time shift and the absorber concentration was observed. The slope of the fitted line gave the molecular extinction coefficient of the absorber. The absorption spectra were also obtained from the time shift. Applicability to dual-wavelength absorption measurement is also discussed. We demonstrate for the first time, as far as we know, the feasibility of absorbance quantification in turbid media by the photon correlation method.     

Autocorrelation Central Hole in Clipped Photon-counting Detection Estimation from the Zero-count Probability Density

Abstract

In this paper we develop a procedure to avoid the autocorrelation central hole that appears in clipped photon-counting detection. This method can be understood as a higher order approximation than the Hofmann procedure. It can be applied in combination with any of the alternative methods to evaluate the autocorrelation function in clipped conditions proposed up until the present. We especially analyse that based on the zero-count probability measurement. Fairly good autocorrelation estimates are obtained for low intensity signals.     

Improved identification of noisy spectra using higher-ordered correlation spectral analysis

We used a higher-order correlation-based method of comparison for spectral identification. Higher-order correlations are an extension of the more familiar second-order cross-correlation function and have the significant advantage of being theoretically shown to eliminate noise of unknown spectral density under certain conditions. Specifically, we applied a third-order correlation technique to the identification of similar IR spectra in the presence of noise. We were able to reduce the effects of noise from a second-order correlation measurement by further processing the measurement with a third-order autocorrelation. Our results showed that the third-order correlation-based method increased the probability of detection of a spectrum in the presence of noise, when compared to using a second-order technique alone. The probability of detection increased enough at low signal-to-noise ratios that this technique may be useful when a second-order correlation technique is not acceptable. The third-order technique is applicable to a single experiment, but improved results were found by averaging the results of multiple experiments.     

Tip-tilt error for extremely large segmented telescopes: detailed theoretical point-spread-function analysis and numerical simulation results

We present an analysis of point-spread functions for segmented mirrors affected by random tip-tilt errors on each segment. In addition to Strehl ratio evaluation, this analysis considers key characteristics such as the intensity and the location of speckles and secondary peaks and the relative energy distribution between these features. We develop a method to describe the shape of a nonaveraged point-spread function and deduce the final expressions for ensemble-averaged characteristics. Based on Keck-type hexagonal segmentation geometry, our study is extended to an arbitrary number of segments, and we describe qualitatively the transition from the case of a mirror with few segments to that of a mirror with several hundred segments--prototype of the next generation of Extremely Large Telescopes.     

The Extension of the Electronic Speckle Photography to the Measurement of In-Plane Displacement

In this paper, a modified speckle photography technique is demonstrated for the measurement of a variable range of the in-plane displacement. The modification focuses on coupling the diode pump solid state laser DPSS with the conventional speckle photography technique. The DPSS laser emerges different wavelengths to provide speckle patterns of suitable size to measure the desired range of the in-plane displacement. The second harmonic generation in a nonlinear crystal of wavelength 532 nm and the principle diode laser wavelength 808 nm are employed in identifying the object positions within a lateral displacement made by a standard linear stage in the range from few microns up to 1.2 mm. The sensitivity and the correlation of the speckles formed by both wavelengths suit both small and large movements. A continues measurement by the modified technique can be achieved by identifying a scale factor in the uniform area in which both wavelengths are effective, and high correlation between the results obtained by 532 and 808 nm is maintained. The uncertainty in measuring 1.2 mm lateral movement by the modified speckle photography is found to be 26.8 μm.     

The Measurement of the First-order Factorial Moment of Double-triggered Photocount Distribution as an Alternative to Triple Correlation

The expression corresponding to the normalized first-order factorial moment (NFOFM) of a double triggered photocount distribution (DTPCD) has been developed. It is found that the NFOFM of a DTCPD has the spectral information of incident light corresponding to the third-order autocorrelation function. A theoretical error model was developed and checked by using a computer simulation method. We also compare the results obtained from the measurement of the NFOFM of a DTPCD with those obtained from the second- and third-order correlation functions.