大功率单色LED的空间相干特性

基于van Cittert-Zernike定理,从理论上计算出大功率单色发光二极管(LED)辐出光,在空间中传输后的空间相干性分布。计算结果表明,自发辐射的LED光源其辐射光在空间中传输后,由非相干光变为部分相干光,且其空间相干性与LED芯片的结构有关。实验上采用双缝干涉对LED辐射光的空间相干性进行测量,由干涉条纹的可见度与两点间的相干度之间的关系得出,非相干LED光源的辐射光在传输后为部分相干光。而采用间隔可调的双缝干涉测量两点间的相干度后发现,LED的芯片发光区域决定了其辐射光在传输中的空间相干性分布。理论计算的空间相干性分布与实验测量结果基本吻合。     

Effects of parylene C layer on high power light emitting diodes

20 μm parylene C layer was deposited on silicone film to enhance the oxygen and water barrier properties, deposited at typical rate of 2.0 and 5.6 μm/h by controlling different deposited pressures. Surface morphology and roughness were observed by atomic force microscopy (AFM), surface morphology images show lower deposited rate that can lead to better quality film. 20 μm parylene C layer was deposited on silicone encapsulation of high power light emitting diodes (LEDs), the samples were tested by power temperature cycling (PTC) test from −40 to 85 °C, 15 min each extreme, and no corrosion, discoloration, cracks was found on the LEDs after the 1000 h PTC reliability test, and PTC test is intended to simulate worst case conditions encountered in typical applications, and parylene C floating membrane structure can stand such high stress and strain. Optical test on white and red LED samples with and without 20 μm parylene C layer, measurement result shows the optical transmittance more than 95%. 1000 h temperature humidity bias life test (T&HB) is performed for the purpose of evaluating the reliability of LEDs in humid environments, energy-dispersive X-ray (EDX) analysis revealed much lower content of C (carbon) and O (oxygen) on the lead frame of LED with parylene C coating after T&HB test on, and no oxidation was found in the LED package.     

杨氏双缝干涉实验中的光谱奇异现象

基于部分相干光的传输理论,研究了杨氏双缝干涉实验中的光谱奇异现象。发现在杨氏双缝干涉实验干涉场区中的某个点的光谱奇异现象,它会随着某些参量(如源光谱宽度Γ′,缝宽参量ε,相对空间相干度Δ0)的变化而改变,指出该现象可应用于信息的编码及自由空间的信息传输。     

部分相干电磁光束在杨氏双缝实验中的偏振特性

推导了部分相干电磁光束经过杨氏双缝干涉后偏振度的表达式,研究了干涉光场偏振度的变化情况.结果表明,干涉光场偏振度不仅取决于双缝处的偏振度、相干长度,而且还取决于杨氏双缝缝宽.当所有相干长度都相等时,干涉光场偏振度不再发生变化,其值就等于双缝处的偏振度.着重分析了系统中交叉谱密度矩阵系数Bxy=Byx≠0的情况,并将结果与Bxy=Byx=0时作了比较.     

Measurement of wire diameter by optical diffraction

A combined interference and diffraction pattern, in the form of equidistant interference fringes, resulting from illuminating a vertical metallic wire by a laser beam is analyzed to measure the diameter of four standard wires. The diameters range from 170 to 450 μm. It is found that the error in the diameter measurements increases for small metallic wires and for small distances between the wire and the screen due to scattering effects. The intensity of the incident laser beam was controlled by a pair of sheet polaroids to minimize the scattered radiation. The used technique is highly sensitive, but requires controlled environmental conditions and absence of vibration effects. The expanded uncertainty for k=2 is calculated and found to decrease from U(D)=±1.45 μm for the wire of nominal diameter 170 μm to ±0.57 μm for the diameter 450 μm.     

Far field spectrum in surface plasmon-assisted Young's double-slit interferometer

We derive an expression for the resultant spectral density (spectrum) at a point in the far zone for the surface plasmons modulated Young's double-slit interference setup. The resultant spectral interference law has the same form as the standard spectral interference law for the scalar fields. This resemblance in turn provides a means for determination of the modified spectral degree of coherence at the two slits. The mathematical results also show that in an interesting situation when the field is incident at one slit only, the interference can still be observed at the observation plane. These findings are verified theoretically using a wide-band source, i.e. a black-body, having a spectrum following Planck's radiation law.     

Direct measurement of transverse coherence length of hard X rays from interference fringes

We propose a simple interferometric technique for hard x-ray spatial coherence characterization, recording a Fresnel interference pattern produced by a round fiber or a slit. We have derived analytical formulas that give a direct relation between a visibility of interference fringes and either the source size or the transverse coherence length. The technique is well suited to third-generation synchrotron radiation sources and was experimentally applied to determine the spatial coherence length and the source size at the European Synchrotron Radiation Facility.     

New data-processing technique for measurement of metallic foil thickness with differential white-light interferometry

The thickness of metallic foil was measured by differential white-light interferometry (DWLI). Two tandem Michelson interferometers (MI), in which the reflective surfaces measured are the surfaces of the metallic foil, were used as a basic interferometry system to obtain interference fringes on a spectrometer. Therefore, the interference fringes depend only on the path differences caused by the thickness of the metallic foil. The interference fringes were analyzed using a modified extremum method based on the least root-mean-square (RMS) deviation. Experimental results for thickness measurements are presented. Measurement time is only 100 ms or less for a thickness of 1–80 μm.     

Resonant-cavity-enhanced photodetectors and LEDs in the mid-infrared

In this paper we outline the use of resonant-cavity enhancement for increasing the exterior coupling efficiency of photodetectors and light-emitting diodes (LEDs) in the mid-infrared (MIR) spectral region. This method is potentially very important in the MIR because encapsulation is not presently feasible due to the lack of suitable materials. Among other potential applications, resonant-cavity-enhanced (RCE) photodetectors and LEDs could be particularly suitable for greenhouse gas detection because of their ‘pre-tunable’ spectrally narrowed resonantly enhanced peaks. We also present the optical characterization of an InAs RCE photodetector aimed at the detection of methane gas (λ≈3.3 μm), and an InAs/InAs_0.91Sb_0.09 resonant-cavity LED (RCLED) aimed at carbon dioxide gas (λ≈4.2 μm). The high peak responsivity of the RCE photodetector was 34.7 A/W at λ=3.14 μm, and the RCLED peaked at λ=3.96 μm. These are among the longest operating wavelengths for III–V RCE photodetectors and RCLEDs reported in the literature.     

Multiple-beam interferometric determination of the coherence length of spectral sources

A theoretical treatment for multiple-beam fringe visibility taking into consideration the profile of the spectral source is derived for (a) a Gaussian profile; and (b) a Lorentzian profile. A comparative study between the visibility of the two profiles is presented. Expressions for the intensity distribution for Doppler and Voigt cases are given. The fringe visibility dependence on the half-width of the spectral profile of the light illuminating the interferometer for the two cases, namely Lorentzian and Gaussian, is presented. This leads to the view that the coherence length for a Voigt profile is less than it is in the absence of Doppler or pressure broadening. Multiple-beam Fabry-Perot fringes in transmission have been utilized for the determination of the coherence length of the two spectral lines emitted from ¹⁹⁸Hg at wavelengths of 5461 Å and 4358 Å. The method for determination is based on the variation of the visibility of the recorded interference fringes with path difference.     

Effects of thermal light source properties in two-photon subwavelength coincidence interference experiments

We perform two-photon coincidence subwavelength interference experiments from double slit using independent photons obtained from a pseudo-thermal light source produced by (i) slowly rotating ground glass (RGG) with turbid solution of a different density and (ii) RGG. The turbid solution is water solution containing 3 μm diameter polystyrene microspheres. It is found that the visibility of the obtained interference pattern decreases in first experiment with increasing the density of the turbid solution from N = 10¹⁰ spheres m⁻³ to N = 10¹⁴ spheres m⁻³. However, the results are reported here for the density of N = 10¹⁴ spheres m⁻³. The visibility obtained with RGG with turbid solution having N = 10¹⁴ spheres m⁻³is 23% which increases to 27% with RGG but the resolution decreases. The effect of coherence width of source in two-photon interference pattern is also studied with pseudo-thermal light obtained by RGG. It was observed that on increasing the coherence width the visibility of interference fringes increased and quality of the fringe reduced and when the coherence width was more then slit separation, in coincidence measurements, no interference pattern appeared. The results are used to explore the classical subwavelength interference nature with thermal light.     

Attosecond double-slit experiment

A new scheme for a double-slit experiment in the time domain is presented. Phase-stabilized few-cycle laser pulses open one to two windows (slits) of attosecond duration for photoionization. Fringes in the angle-resolved energy spectrum of varying visibility depending on the degree of which-way information are measured. A situation in which one and the same electron encounters a single and a double slit at the same time is observed. The investigation of the fringes makes possible interferometry on the attosecond time scale. From the number of visible fringes, for example, one derives that the slits are extended over about 500 as.     

Parameter estimation by decoherence in the double-slit experiment

We discuss a parameter estimation problem using quantum decoherence in the double-slit interferometer. We consider a particle coupled to a massive scalar field after the particle passing through the double slit and solve the dynamics non-perturbatively for the coupling by the WKB approximation. This allows us to analyze the estimation problem which cannot be treated by master equation used in the research of quantum probe. In this model, the scalar field reduces the interference fringes of the particle and the fringe pattern depends on the field mass and coupling. To evaluate the contrast and the estimation precision obtained from the pattern, we introduce the interferometric visibility and the Fisher information matrix of the field mass and coupling. For the fringe pattern observed on the distant screen, we derive a simple relation between the visibility and the Fisher matrix. Also, focusing on the estimation precision of the mass, we find that the Fisher information characterizes the wave-particle duality in the double-slit interferometer.     

Transverse spatial coherence of a transient nickellike silver soft-x-ray laser pumped by a single picosecond laser pulse

The degree of spatial coherence in the direction perpendicular to the target surface is reported for a transient nickellike silver x-ray laser at 13.9 nm. An x-ray laser plasma column was produced by irradiating a slab silver target with a single shaped picosecond laser pulse with energy less than 3 J. Young's double-slit method was applied to measure the fringe visibility as a function of the slit separation for different target lengths. The diameter of the equivalent incoherent source and the coherence radius of the output radiation were determined as well.     

Spatial partial coherence in Young''s interferograms

Young's interferograms with high visibility reveals a high degree of spatial coherence of first order. But, spatially partial coherence of second order can be observed when it interferes itself through a compensated Michelson's interferometer attached at the exit of the Young's slit pair. We show that the patterns at the exit of the Michelson's interferometer are Young's interferograms with modulation fringes, which allow an estimation of the degree of the high order spatial coherence.     

Coherent and incoherent spectral broadening in a photonic crystal fiber

The coherence of the spectral broadening process is the key requisite for the application of supercontinua in frequency combs. We investigate the coherence of two subsequent supercontinuum pulses created in a photonic crystal fiber pumped by a femtosecond laser. We measure Young interference fringes from a Michelson-type interferometer at different wavelengths of the output spectrum and analyze their dependence on pump intensity and polarization. The visibility of these fringes is a direct measure of the coherence of the spectral broadening processes.     

Contrasts of Stokes parameters in Young's interference experiment and electromagnetic degree of coherence

We analyze the modulation of the Stokes parameters in Young's two-pinhole interference experiment with a random electromagnetic beam. We demonstrate that the electromagnetic (spectral) degree of coherence put forward in Opt. Lett.29, 328 (2004) [or its space-time analog in Opt. Express 11, 1137 (2003)] is physically related to the contrasts of modulation in the four Stokes parameters. More explicitly, the electromagnetic degree of coherence is a measure of both the visibility of the intensity fringes and the modulation contrasts of the three polarization Stokes parameters. We also show that by using suitable wave plates the modulation in any Stokes parameter can be transformed into the form of intensity variation, and hence the electromagnetic degree of coherence can be obtained experimentally by four visibility measurements.     

Surface plasmons modulate the spatial coherence of light in Young's interference experiment

It is shown how surface plasmons that travel between the slits in Young's interference experiment can change the state of spatial coherence of the field that is radiated by the two apertures. Surprisingly, the coherence can both be increased and decreased, depending on the slit separation distance. This results in a modulation of the visibility of the interference fringes. Since many properties of a light field-such as its spectrum, polarization, and directionality - may change on propagation and are dependent on the spatial coherence of the source, our results suggest that the use of surface plasmons provides a new way to alter or even tailor the statistical properties of a light field.     

Simultaneous spectral calibration of two phase plates

Based on a new model for the spectral calibration of phase plates in pairs, experimental results are presented for three mica plates between 430 and 684 nm. One of the plates is 36.4 μm thick and the remaining two are identical, each being 28.6 μm thick. In the studied spectral range, the retardances varied from 123.5° to 86.8° for the first plate and from 96.9° to 68° for each of the two other plates. Different procedures for performing the calibration of the plates and the special case of two identical plates are discussed.     

A new formula for fringe localization in holographic interferometry

Holographic interferometry generally involves the interference of two or more speckle fields reflected from or transmitted through a diffused object. The visibility of the resulting fringes depends on the degree of coherence of the interfering speckle fields. Through a statistical analysis it is found that the degree of coherence of speckle fields is essentially a function of the relative speckle displacement and average speckle size. The analysis leads to a new physical model for holographic interferometry. With this model the problem of fringe formation and localization may be analyzed and formulated in terms of speckle movement. A new formula for ideal fringe localization is then derived from the analysis. Furthermore, a localization factor V_c, i.e. the visibility of the holographic fringes, is introduced to analyze the partial fringe localization in holographic interferometry, which is of practical interest to most researchers.