The influence of nonparaxiality on the spectral behavior in Young's experiment illuminated by partially coherent light

Starting from the Rayleigh-Sommerfeld diffraction integral, this paper studies the spectral behavior in Young＇s experiment illuminated by nonparaxial partially coherent light and compares with the paraxial case, where the influence of nonparaxiality of partially coherent light on the spectral shifts and spectral switches is stressed. It is shown that there is a spectral shift in the nonparaxial case relative to the paraxial one and the critical position changes, at which the spectral switch occurs. The ratio of the waist width to the central wavelength ω0/λ0 and relative spatial correlation length △ affect the spectral difference. The smaller ω0/λ0 is, the larger the difference between the nonparaxial and paraxial results appears. The effect of relative spatial correlation length △ is relatively small.

MEASURING DISPLACEMENT FIELDS OF CURVED SURFACE BY MOIRE INTERFEROMETRY OF PARTIAL COHERENT LIGHT

This paper presents a method to measure the in-plane displacement fields of curved surface by moire interferometry of partial coherent light.The method has the following advantages:simple optical system,no requirement on vibration isolation,high sensitivity,large measuring range,high contrast of inter ference fringes and availability to in-situ structural testing.The present paper also gives theoretical analysis of the method and the formulas of light intensity and displacement field,and introduces a replication technique to form a high frequency reflectance grating on the curved surface.The experiments achieved the measurement of the surface displacement field of a cylindrical shell—the simultaneous circumferential,axial and 45° displacement fields.The torsional test data for surface displacement of a circular bar agree well with the theoretical result.

At wavelength coherent scatterometry microscope using high-order harmonics for EUV mask inspection

In this review,we describe our research on the development of the 13.5 nm coherent microscope using high-order harmonics for the mask inspection of extreme ultraviolet(EUV)lithography.EUV lithography is a game-changing piece of technology for high-volume manufacturing of commercial semiconductors.Many top manufacturers apply EUV technology for fabricating the most critical layers of 7 nm chips.Fabrication and inspection of defect-free masks,however,still remain critical issues in EUV technology.Thus,in our pursuit for a resolution,we have developed the coherent EUV scatterometry microscope(CSM)system with a synchrotron radiation(SR)source to establish the actinic metrology,along with inspection algorithms.The intensity and phase images of patterned EUV masks were reconstructed from diffraction patterns using ptychography algorithms.To expedite the practical application of the CSM,we have also developed a standalone CSM,based on high-order harmonic generation,as an alternative to the SR-CSM.Since the application of a coherent 13.5 nm harmonic enabled the production of a high contrast diffraction pattern,diffraction patterns of sub-100 ns size defects in a 2D periodic pattern mask could be observed.Reconstruction of intensity and phase images from diffraction patterns were also performed for a periodic line-and-space structure,an aperiodic angle edge structure,as well as a cross pattern in an EUV mask.

Coherent free-electron light sources

Free-electron light sources feature extraordinary luminosity,directionality,and coherence,which has enabled significant scientific progress in fields including physics,chemistry,and biology.The next generation of light sources has aimed at compact radiation sources driven by free electrons,with the advantages of reduction in both space and cost.With the rapid development of ultra-intense and ultrashort lasers,great effort has been devoted to the quest for compact free-electron lasers(FELs).This review focuses on the current efforts and advancements in the development of compact FELs,with a particular emphasis on two notable paths:the development of compact accelerators and the construction of micro undulators based on innovative materials/structures or optical modulation of electrons.In addition,the physical essence of inverse Compton scattering is discussed,which offers remarkable capability to develop an optical undulator with a spatial period that matches the optical wavelength.Recent scientific developments and future directions for miniaturized and integrated free-electron coherent light sources are also reviewed.In the future,the prospect of generating ultrashort electron pulses will provide fascinating means of producing superradiant radiation,promising high brilliance and coherence even on a micro scale using optical micro undulators.

Spectral anomalies of polychromatic, spatially coherent light diffracted by an annular aperture in the far field

The spectral behavior of polychromatic spatially fully coherent light diffracted by an annular aperture in the far field is studied. It is shown that the spectrum in the far field is generally different from that at the aperture, i.e., the spectrum in the far field is proportional to the spectrum at the aperture and a spectral modifier, which depends on the central obstruction ratio ε and diffraction angle θ. Detailed numerical calculation results indicate that significant spectral changes take place in the vicinity of zeros of the Airy pattern. It is found that at the critical diffraction angle θc, the spectrum is split into two lines, while at a diffraction angle a little smaller than θc, the spectrum is red-shifted, and at a diffraction angle a little larger than θc, the spectrum is blue-shifted. The influence of the central obstruction ratio on the spectral anomalies is presented.

Spectral anomalies and spectral switches of partially coherent and polychromatic light diffracted at an aperture

Based on the propagation theory of partially coherent light in the space-frequency domain, the anomalous spectral behavior and spectral switches in the far field of partially coherent and polychromatic light diffracted at an aperture are studied. It is shown that, as compared with spatially fully coherent and polychromatic light whose spectral anomalies are induced only by aperture diffraction, the spectral anomalies and spectral switches of partially coherent and polychromatic light depend on the aperture diffraction, spatial correlationβ and bandwidth of the original spectrum. Detailed numerical calculations are made to illustrate the behavior of spectral switches of partially coherent and polychromatic light, and the results for spatially fully coherent and polychromatic light are treated as a special case ofβ=1 and included in our theory.

Quantum field entropy of the system with interaction between moving Bell-state atoms and coherent light field

The field entropy of the system with two moving atoms interacting with the coherent state is investigated by means of the full quantum theory. Under the different initial states with two atoms, the influences of the light field intensity and the atomic motion on the field entropy are discussed. The results indicate that the motion of the atoms leads to strict periodicity in the field entropy evolution. When the two atoms are in the Bell state ｜β11〉 initially, the system is in a completely disentangled state. For the atoms initially at other Bell states, the field periodically entangles with the atoms.

Entropy of field interacting with two two-qubit atoms

We use quantum field entropy to measure the degree of entanglement for a coherent state light field interacting with two atoms that are initially in an arbitrary two-qubit state. The influence of different mean photon number of the coherent field on the entropy of the field is discussed in detail when the two atoms are initially in one superposition state of the Bell states. The results show that the mean photon number of the light field can regulate the quantum entanglement between the atoms and light field.

Event-by-Event Simulation of the Hanbury Brown-Twiss Experiment with Coherent Light

We present a computer simulation model for the Hanbury Brown-Twiss experiment that is entirely particle-based and reproduces the results of wave theory. Themodel is solely based on experimental facts, satisfies Einstein’s criterion of local causality and does not require knowledge of the solution of a wave equation. The simulationmodel is fully consistent with earlier work and provides another demonstration thatit is possible to give a particle-only description of wave phenomena, rendering theconcept of wave-particle duality superfluous.

New proposal of modulation of amplitude-squeezed state of light by intensity variation of a low-frequency coherent message signal

Squeezed state of light explores a new era in noiseless communication and data processing recently breaking the quantum limit of noise. We propose a new mechanism of modulating an amplitude-squeezed signal with the instantaneous intensity variation of a coherent signal. The modulating signal is a coherent light where the amplitude-squeezed light takes the role of a carrier signal.

Optical coherent detection Brillouin distributed optical fiber sensor based on orthogonal polarization diversity reception

In Brillouin distributed optical fiber sensor, using optical coherent detection to detect Brillouin scattering optical signal is a good method, but there exists the polarization correlated detection problem. A novel detecting scheme is presented and demonstrated experimentally, which adopts orthogonal polarization diversity reception to resolve the polarization correlated detection problem. A laser is used as pump and reference light sources, a microwave electric-optical modulator (EOM) is adopted to produce frequency shift reference light, a polarization controller is used to control the polarization of the reference light which is changed into two orthogonal polarization for two adjacent acquisition periods. The Brillouin scattering light is coherently detected with the reference light, and the Brillouin scattering optical signal is taken out based on Brillouin frequency shift. After electronic processing, better Brillouin distributed sensing signal is obtained. A 25-km Brillouin distributed optical fi

Partially coherent vortex beams propagation in a turbulent atmosphere

Based on the Rytov approximation and the cross-spectral density approximation for the mutual coherence function of the partially coherent field, the propagation properties of the partially coherent beams with optical vortices in turbulent atmosphere are discussed. The average intensity and the mutual coherence function of the partially coherent vortex beams propagation in weak turbulent atmosphere are obtained. It is shown that the vortex structure of the average cross-spectral density of partially coherent beams has the same helicoidally shape as that of the phase of the fully coherent Laguerre-Gauss beams in free space and the relative intensity of the beam is degraded by optical vortex.

Visible light optical coherence tomography measures retinal oxygen metabolic response to systemic oxygenation

The lack of capability to quantify oxygen metabolism noninvasively impedes both fundamental investigation and clinical diagnosis of a wide spectrum of diseases including all the major blinding diseases such as age-related macular degeneration,diabetic retinopathy,and glaucoma.Using visible light optical coherence tomography(vis-OCT),we demonstrated accurate and robust measurement of retinal oxygen metabolic rate(rMRO2)noninvasively in rat eyes.We continuously monitored the regulatory response of oxygen consumption to a progressive hypoxic challenge.We found that both oxygen delivery,and rMRO2 increased from the highly regulated retinal circulation(RC)under hypoxia,by 0.2860.08 μL min(^-1)(p,0.001),and 0.2060.04 μL min(^-1)(p,0.001)per 100 mmHg systemic pO2 reduction,respectively.The increased oxygen extraction compensated for the deficient oxygen supply from the poorly regulated choroidal circulation.Results from an oxygen diffusion model based on previous oxygen electrode measurements corroborated our in vivo observations.We believe that vis-OCT has the potential to reveal the fundamental role of oxygen metabolism in various retinal diseases.

Variation analysis of turbulence resistance and angular spreading for partially coherent beam in turbulence

A new factor M is proposed to characterize the similarity of the behavior of a partially coherent beam （PCB） to its coherent counterpart when propagating through atmospheric turbulence. It is shown that there exists a boundary in the range of the source coherent length. The decreasing rates of free space angular spreading and of turbulence distance with the source coherent length are different before and after the coherent length arriving at the boundary value, by which the change trend of M can be concluded.

Photon statistics measurement for coherent fields

An efficient scheme for photon statistics measurement is presented based on the Hanbury-Brown-Twiss configuration. We set the sampling time Ts to satisfy the relationship of Ts 〈 Td 〈 Tm, where Td is the dead time of each detector and Tm is the laser pulse repetition period. And each single photon detector cannot detect more than one photon in each pulse. The approach can sufficiently eliminate the influences of the detector＇s dead time on photon statistics. At last, the photon statistics of coherent field is experimentally determined.

Measurement of the degree of temporal coherence of unpolarized light beams

We measure the electromagnetic degree of temporal coherence and the associated coherence time for quasi-monochromatic unpolarized light beams emitted by an LED, a filtered halogen lamp, and a multimode He–Ne laser.The method is based on observing at the output of a Michelson interferometer the visibilities(contrasts) of the intensity and polarization-state modulations expressed in terms of the Stokes parameters. The results are in good agreement with those deduced directly from the source spectra. The measurements are repeated after passing the beams through a linear polarizer so as to elucidate the role of polarization in electromagnetic coherence. While the polarizer varies the equal-time degree of coherence consistently with the theoretical predictions and alters the inner structure of the coherence matrix, the coherence time remains almost unchanged when the light varies from unpolarized to polarized. The results are important in the areas of applications dealing with physical optics and electromagnetic interference.

Damage characteristics of fused silica under low-temporal coherence light

The damage characteristics of fused silica were investigated under low-temporal coherence light(LTCL).It was found that the laser-induced damage threshold(LIDT)of fused silica for the LTCL was lower than that of the single longitudinal mode pulse laser,and for the LTCLs,the LIDTs decrease with the increasing of laser bandwidth,which is not consistent with the temporal spike intensity.This is due to the nonlinear self-focusing effect and multi-pulse accumulation effect.The specific reasons were analyzed based on theoretical simulation and experimental study.This research work is helpful and of great significance for the construction of high-power LTCL devices.

Slant path average intensity of finite optical beam propagating in turbulent atmosphere

The average intensity of finite laser beam propagating through turbulent atmosphere is calculated from the extended Huygens Fresnel principle. Formulas are presented for the slant path average intensity from an arbitrarily truncated Gaussian beam. The new expressions are derived from the modified von Karman spectrum for refractive-index fluctuations, quadratic approximation of the structure function, and Gaussian approximation for the product of Gaussian function and Bessel function. It is shown that the form of average intensity is not a Gaussian function but a polynomial of the power of the binomial function, Gaussian function, and the incomplete gamma function. The results also show that the mean irradiance of a finite optical beam propagating in slant path turbulent atmosphere not only depends on the effective beam radius at the transmitting aperture plane, propagation distance, and long-term lateral coherence length of spherical wave, but also on the radius of emit aperture.

Using OCT image to distinguish human acupoint from non-acupoint tissues after irradiation with laser in vivo:a pilot study

Using the optical coherence tomography（OCT） images,the optical attenuation coefficients（μ_t） of human Laogong acupoint and non-acupoint tissues are measured after empty irradiation and 808-nm 100-mW irradiation for 10 min in vivo non-invasively.The results show that there is no significant difference ofμ_t of Laogong acupoint and non-acupoint tissues after empty irradiation.However,there are significant differences ofμ_t between Laogong acupoint and non-acupoint tissues after laser irradiation at the power of 100 mW（statistical definition of probability p0.01）.The results of the pilot study indicate that the OCT could distinguish the acupoint from the surrounding tissues after irradiation with laser in vivo non-invasively.

Measurement of crater geometries after laser ablation of bone tissue with optical coherence tomography

The feasibility of measuring crater geometries by use of optical coherence tomography （OCT） is examined. Bovine shank bone on a motorized translation stage with a motion velocity of 3 mm/s is ablated with a pulsed CO2 laser in vitro. The laser pulse repetition rate is 60 Hz and the spot size on the tissue surface is 0.5 mm. Crater geometries are evaluated immediately by both OCT and histology methods after laser irradiation. The results reveal that OCT is capable of measuring crater geometries rapidly and noninvasively as compared to histology. There are good correlation and agreement between crater depth estimates obtained by two techniques, whereas there exists distinct difference between crater width estimates when the carbonization at the sides of craters is not removed.