Jixiong Pu

Spectral shifts and spectral switches of partially coherent light passing through an aperture

Within the Fresnel approximation, we investigate the spectral characteristics in the near zone when a certain class of partially coherent light is incident on a circular aperture. It is shown that, as the distance z between the aperture and the on-axis observation point decreases, the spectral shifts show gradual change. However, when z approaches a critical value, a rapid spectral switch occurs. The evolution from a gradual spectral shift to a rapid spectral switch is illustrated. The effect of the coherence on the spectral switch is also presented.

Tight focusing of a double-ring-shaped, azimuthally polarized beam

We study the focusing properties of a double-ring-shaped azimuthally polarized beam through an annular high NA objective lens. It is shown that a subwavelength focal hole (∼0.5λ) with a quite long depth of focus (∼26λ) is achieved near the focus. This kind of nondiffracting focal hole is called dark channel, which may have applications in atom optical experiments, such as with atomic lenses, atom traps, and atom switches.

Focusing of partially coherent Bessel-Gaussian beams through a high-numerical-aperture objective

The focusing properties of partially coherent Bessel-Gaussian beams passing through a high-numerical-aperture objective are studied based on vectorial Debye theory. Expressions for the intensity distribution, degree of coherence |mu|, and degree of polarization P are derived near the focus. Numerical calculations are performed to analyze the influences of varying corresponding parameters on the intensity distribution, |mu|, and P in the focal region. It is shown that the intensity, |mu|, and P in the focal region are all influenced by varying the effective coherent length L(c) of incident beams and maximal angle alpha determined by the numerical aperture of the objective. Also, the linearly polarized incident field is found to be depolarized after it is focused by a high-numerical-aperture objective.

Spectral anomalies in Young’s double-slit interference experiment

We report a phenomenon of spectral anomalies in the interference field of Youngs double-slit interference experiment. The potential applications of the spectral anomalies in the information encoding and information transmission in free space are also considered.

Spectral shifts and spectral switches in diffraction of partially coherent light by a circular aperture

When a particular class of partially coherent light is incident upon a circular aperture, the on-axis spectrum of the light in the far zone is different from the spectrum of the light at the aperture. An explicit expression describing the on-axis spectrum of the light in the far zone is obtained and the numerical calculation is performed. It is shown that by measuring the spectral shifts against some parameter, such as a/L~(a is the radius of the aperture and L~ is the effective correlation length of the light at the aperture at the central frequency of the source spectrum); we find that the spectral shifts show a gradual change. However when a/L~ is equal to particular values, the spectral shifts exhibit a rapid transition, i.e., spectral switches occur. Some parameters, such as the half-width at half-maximum of the source spectrum /spl Gamma/ and the ratio of the central obstruction of the source /spl epsiv//sub s/ affect the spectral switch positions and the spectral switch performance. The generation of multi-spectral lines in the far zone from a single spectral line is also shown.

Invariance and noninvariance of the spectra of stochastic electromagnetic beams on propogation

It has been known for some time that the spectrum of light may change on propagation, even in free space. The theory of this phenomenon was developed within the framework of scalar theory. We generalize it to electromagnetic beams, generated by planar, secondary, stochastic sources. We also derive an electromagnetic analog of the so-called scaling law. When this law is satisfied the normalized spectrum of the beam is the same throughout the far zone and across the source.

Investigation on the scintillation reduction of elliptical vortex beams propagating in atmospheric turbulence.

We numerically investigate the scintillation index of an elliptical vortex beam in both modest turbulence and strong turbulence. Numerical simulations are realized with random phase screen scheme. It is shown that the on-axis scintillation index can be effectively reduced by an elliptical vortex beam if crucial parameters are properly chosen. The mechanisms of scintillation reduction in turbulence of different strengths are different. We find that the topological charge and the ratio of minor axis to major axis of an elliptical vortex beam are important in determining the on-axis scintillation index. Our simulation results indicate that using an elliptical vortex beam is a promising strategy to alleviate atmospheric influence on free space optical communication link.

Spectral changes and 1 X N spectral switches in the diffraction of partially coherent light by an aperture.

The off-axis and on-axis spectra in the far zone of an aperture for the case in which a particular class of partially coherent light with a broad spectrum is diffracted by an aperture are studied. It is shown that the spectrum in the far zone is generally different from that at the aperture; i.e., the spectrum is split into two or more peaks. Moreover, the spectrum varies with the diffractive angle. For a fixed diffractive angle, the spectral shift, defined as the difference between the frequencies at which the observed spectrum and the spectrum at the aperture take their maximum, shows a gradual change with the change in the coherence at the aperture. However, as the coherence reaches some critical values, the spectral shift exhibits a rapid transition; i.e., spectral switch occurs. The coherence that causes the spectral switch to take place is different for different diffractive angles. Therefore we propose a new kind of 1 x N spectral switch, where N detectors (output ports) are placed at different diffractive angles in the far zone, and the spectral shifts at different detectors are measured. By adjusting the coherence of the aperture (input port) to the desired values, we obtain a rapid transition of the spectral shift in the desired output ports.

Propagation characteristics of aberrant stochastic electromagnetic beams in a turbulent atmosphere

In this paper, we investigate the characteristics of stochastic electromagnetic beams with astigmatic aberration propagating through a turbulent atmosphere. Taking the electromagnetic Gaussian Schell-model beam (GSM beam) as an example, the analytic expressions for the intensity and the degree of polarization of the beams propagating through the turbulent atmosphere are derived. It is shown that the intensity distribution and the degree of polarization of the GSM beam in the turbulent atmosphere are severely influenced by the astigmatism coefficient, as well as by the correlation property and the atmospheric turbulence. We show that when beams with astigmatic aberration propagate in a turbulent atmosphere, the beams become asymmetric in the near field and turn out to be symmetric after a quite long distance propagation in the turbulent atmosphere.

Propagation of partially coherent vortex beams in a turbulent atmosphere

A partially coherent vortex beam propagating in a turbulent atmosphere is investigated by using extended Huygens-Fresnel diffrac- tion integral. The influence of the turbulence, the spatial coherence, and the topological charges of the incident beam on the propagation charac- teristics of the beam is studied in great detail. It is shown that the spread- ing of the partially coherent vortex beam is less influenced by atmo- spheric turbulence than that of a partially coherent nonvortex beam, and that a partially coherent vortex beam of lower coherence is less influ- enced than one of higher coherence.