Wavefront-folding Interferometer Research Articles

Specular transformation of electromagnetic partially coherent beams with a wave-folding interferometer

We consider a class of specular or anti-specular vector beams, by illuminating stochastic vector beams into a prism-based wavefront-folding interferometer. Such transform is applied to various genuine model input beams, and then the properties of the resulting fields are discussed. Numerical results show that the specular nature of these vector fields not only creates sharp internal spectral density distribution, but also produces novel polarization patterns with oscillations or a central dip on the degree of polarization-profile. Such optical characteristics can be flexibly modulated by the correlation structure of the source. We also suggest that the specular transform could be efficiently employed in developing novel partially coherent vector beams.

Propagation of Bessel-correlated specular and antispecular beams.

We address the specular properties of Bessel-correlated fields, generated by illuminating a tilted rotating plane-parallel glass plate with a coherent Gaussian beam and passing the output beam though a mirror-based wavefront folding interferometer. This device allows us to produce beams whose specular properties are preserved in propagation. In the far zone, the specular nature of these partially coherent fields is shown to produce intensity-profile oscillations in the sub-diffraction-limit scale. The analytical results at various propagation distances are verified experimentally by using another wavefront-folding interferometer for coherence measurements.

Transformation of asymmetric Schell-model beams with a wavefront-folding interferometer

We consider a class of random light beams, which are generated by inserting an asymmetric Schell-model beam into a wavefront-folding interferometer. Typical propagation behavior of such optical beams modulated by several legitimate complex coherence states has been investigated. In certain cases, these beams exhibit novel rotationally symmetric intensity patterns with rectangular or lattice-like lobes, and the pattern and location of each lobe can be controlled by adjusting the source coherence parameters. Besides, beam characteristics can be flexibly modulated by varying the phase delay between the two paths of the interferometer. Our results may be applied in free-space optical communication.

Mirror-based scanning wavefront-folding interferometer for coherence measurements.

We demonstrate a modification to the traditional prism-based wavefront-folding interferometer that allows the measurement of spatial and temporal coherence, free of distortions and diffraction caused by the prism corners. In our modified system, the two prisms of the conventional system are replaced with six mirrors. The whole system is mounted on a linear XY-translation stage, with an additional linear stage in the horizontal arm. This system enables rapid and exact measurement of the full four-dimensional degree of coherence, even for relatively weak signals. The capabilities of our system are demonstrated by measuring the spatial coherence of two inhomogeneous and non-Schell model light sources with distinct characteristics.

Generating dark and antidark beams using the genuine cross-spectral density function criterion.

In this work, we demonstrate how to generate dark and antidark beams-diffraction-free partially coherent sources-using the genuine cross-spectral density function criterion. These beams have been realized in prior work using the source's coherent-mode representation and by transforming a J0-Bessel correlated partially coherent source using a wavefront-folding interferometer. We generalize the traditional dark and antidark beams to produce higher-order sources, which have not been realized. We simulate the generation of these beams and compare the results to the corresponding theoretical predictions. The simulated results are found to be in excellent agreement with theory, thus validating our analysis. We discuss the pros and cons of our synthesis approach vis-à-vis the prior coherent modes work. Lastly, we conclude this paper with a brief summary, and a discussion of how to physically realize these beams and potential applications.

Polarization properties of stochastic electromagnetic beams modulated by a wavefront-folding interferometer.

We consider the field generated by a wavefront-folding interferometer which is illuminated by a stochastic electromagnetic beam. The specular property and anti-specular property are discussed in the vector case. Take electromagnetic Gaussian Schell-model beam as an example, we investigate the spectral density, the spectral degree of coherence, the spectral degree of polarization as well as the state of polarization of the polarized portion of the field on propagation. Results show that the polarization properties including the degree of polarization, the orientation angle and the degree of ellipse can be adjusted by the phase difference of the interferometer and the phase retardation introduced by the prism. The results may be applied in free-space optical communication.

Bessel-correlated supercontinuum fields.

We examine the spatial coherence properties of supercontinuum fields generated by illuminating rotating bulk media with intense pulsed beams. Theoretical models are presented, which indicate the possibility of generating a class of Bessel-correlated fields (in time-averaged sense) using tilted plane-parallel glass plates and wedges as media for generation of supercontinuum radiation. In special cases, the ensuing fields have a strictly identical functional form in the spatial and angular domains. Some of the main results are verified experimentally by measuring the spatial coherence properties of bulk-generated supercontinuum fields using a wavefront-folding interferometer.

Interfering optical coherence lattices by use of a wavefront-folding interferometer.

We investigate the properties of an interfering optical coherence lattice, which is generated by passing an optical coherence lattice through a wavefront-folding interferometer. Two symmetrical lattices are formed in the far-field. Changing the phase difference between the two optical paths causes the interference pattern, and thus the far-field intensity profile to change. In addition, we have shown that in the non-uniform case, the intensity pattern can also be changed by varying the weight distribution parameter. Our results may be applied in free-space communications and periodic trapping of micro-particles.

Influence of atmospheric turbulence on the properties of specular and antispecular beams.

A class of optical fields with specular or antispecular properties can be generated by a Gaussian Schell-model beam passing through a wavefront-folding interferometer. Based on the generalized diffraction integral formula, an analytical expression for the cross-spectral density function of such fields propagating through non-Kolmogorov atmospheric turbulence is derived. It is revealed that the specular and antispecular properties of the beams always maintain during propagation in free space. However, the specularity and antispecularity properties of the beams become different in atmosphere, since they are quickly destroyed by the atmospheric turbulence.

Digital generation of partially coherent vortex beams.

We present an experimental technique to generate partially coherent vortex beams with an arbitrary azimuthal index using only a spatial light modulator. Our approach is based on digitally simulating the intrinsic randomness of broadband light passing through a spiral phase plate. We illustrate the versatility of the technique by generating partially coherent beams with different coherence lengths and orbital angular momentum content, without any moving optical device. Consequently, we study its cross-correlation function in a wavefront folding interferometer. The comparison with theoretical predictions yields excellent agreement.

Changes in radiation forces acting on a Rayleigh dielectric sphere by use of a wavefront-folding interferometer.

We consider a class of fields generated by passing an isotropic Gaussian Schell-model beam through a wavefront-folding interferometer. The output field has various intensity profiles for different phase differences, including the central peak and doughnut shapes. The radiation force on a Rayleigh dielectric particle produced by the highly focused fields is investigated. Numerical results demonstrate that the new fields can be used to trap high-index particles at the focus for the specular case and nearby the focus for the anti-specular case. It is further revealed that the position, the range of particle sizes and the low limit of correlation length for stable trapping could be modulated by adjusting the phase difference.

Specular and antispecular light beams

We consider a class of spatially partially coherent light beams, which are generated by passing a Gaussian Schell-model beam though a wavefront-folding interferometer. In certain cases these beams are shape-invariant on propagation and can exhibit sharp internal structure with a central peak (specular beam) or a central dip (antispecular beam) whose dimensions depend on the spatial coherence area. Such beams are demonstrated experimentally and their cross-like distributions of the complex degree of spatial coherence are measured with a digital micromirror device.

Noise-Limitations of Coherence Imaging Based on Wavefront Folding Interferometry

Noise-limitations of the coherence imaging system based on the wavefront folding interferometer are studied. The signal-to-noise ratio SNR in two kinds of noise-limiting cases, the photon-noise-limit and the detector-noise-limit, is derived and are compared with the experimental results. The experimental demonstration is conducted using a wavefront folding interferometer and a single slit that provides the light source. An ensemble of ten sets of data are measured under the same conditions and the statistics of the retrieved images are computed from them. It is verified that the experimental results generally agree with the theoretical expectations.

Interferometric image reconstruction through the turbulent atmosphere

A high-resolution image of an incoherent object can be recovered in spite of the atmospheric turbulence. The spatial frequency spectrum of object radiance is calculated from digitized interference patterns obtained by a computer-controlled wave front folding interferometer. The reconstruction technique and its application to a laboratory-generated turbulence are presented.

Spatial coherence measurements through turbulent atmosphere using a computer-aided interferometer

The complex degree of coherence over a telescope aperture is measured by processing instantaneous values of the complex degree of coherence fluctuated by turbulent atmosphere. Fringe patterns produced by a computer-controlled wave-front folding interferometer are analyzed in a short time by a photodiode array to calculate the instantaneous values of the complex degree of coherence. The system is applied to one-dimensional simulation experiments.

An achromatic wavefront folding interferometer

A wavelength compensated wavefront folding interferometer is described which produces over 200 achromatic fringes from extended polychromatic objects.

Measurement of the amplitude of phase excursions in the earth’s atmosphere

The maximum of the probability density distribution of phase fluctuations in the earth’s atmosphere, as a function of point separation, was measured visually in white light for starlight observed from the ground with a 1.5 m astronomical telescope. A wave-front folding interferometer was used.