Measurements of complex degree of spatial coherence at the end face of an optical fiber

The phase and modulus of the complex degree of spatial coherence at the exit end face of an optical fiber are measured with the help of a real-time data processing computer. Interference fringes produced by superimposing wavefronts of the image of the end cross-section are scanned by a linear photodiode array to calculate the one dimensional value of spatial coherence. Evolution of phase measured at the different positions in the fringe pattern reveals that the phase is intimately related to modal structures of the propagating field in an optical fiber supporting a small number of guided modes.     

Interference phenomenon on the complex degree of coherence

Optical Review - A two-beam interferometer (TBI) with a two-mode fiber (TMF) as a dispersive medium in one arm and air in the other provides evidence of interference related to the complex degree...     

On the coherence length of a monochromator for use with a michelson stellar interferometer

The drop in visibility due to path difference errors in a Michelson stellar interferometer is normally controlled by a monochromator in the system. It is shown that the effect of this monochromator on the visibility is governed by a product of two factors, one geometrical and the other a function of the resolving power of the monochromator. The second factor causes the visibility to vanish identically for path differences exceeding a certain value. A comparison of prisms and gratings shows that in general a prism train is preferable as a dispersing device.     

Measuring spatial coherence by using a reversed-wavefront Young interferometer

A very simple optical setup for the measurement of the modulus and the phase of the two-point correlation function of a partially coherent light field is presented. The system consists of a slightly modified version of a Young interferometer and requires a single Young mask in order to determine the correlation function at any pairs of points. Experimental results are presented for the case of a synthesized partially coherent secondary source.     

Retrieving the complex degree of spatial coherence of electron beams

The possibility to characterize the coherence properties of an electron source is presented. The method, based on the determination of centered-reduced moments of the beam spot, allows the evaluation of both amplitude and phase of the complex degree of spatial coherence. The experimental results are in agreement with a different approach based on the Fourier analysis and with calculations according to the Van Cittert—Zernike theorem.     

Condition for invariant spectral degree of coherence of an electromagnetic plane wave on scattering

Within the accuracy of the first-order Born approximation, the condition for invariant spectral degree of coherence of an electromagnetic plane wave scattered from random media is presented. The condition for the electromagnetic plane wave is different from the one for the scalar plane wave. Results also indicate that, different polarizations of the incident plane wave would have essential effects on analytical forms of the condition. These effects may be due to the correlation-induced changes in the scattered spectral degree of coherence.     

Full range complex spectral optical coherence tomography technique in eye imaging

We demonstrate a new implementation of complex spectral optical coherence tomography (OCT) in biomedical imaging. By reconstruction of both amplitude and phase we are able to use the negative and positive optical path differences to get images of objects of considerable thickness. An accompanying reduction of coherent noise improves the quality of the images. The property of the complex spectral OCT that permits the measurement range to be increased and permits the simultaneous use of phase and amplitude in spectral systems was not described previously. To show the potential of this technique we measured an anterior chamber of a porcine eye in vitro.     

Real-time in vivo imaging of human gastrointestinal ultrastructure by use of endoscopic optical coherence tomography with a novel efficient interferometer design

We report on the design and initial clinical experience with a real-time endoscopic optical coherence tomography (EOCT) imaging system. The EOCT unit includes a high-speed optical coherence tomography interferometer, endoscope-compatible catheter probes, and real-time data capture and display hardware and software. Several technological innovations are introduced that improve EOCT efficiency and performance. In initial clinical studies using the EOCT system, the esophagus, stomach, duodenum, ileum, colon, and rectum of patients with normal endoscopic findings were examined. In these initial investigations, EOCT imaging clearly delineated the substructure of the mucosa and submucosa in several gastrointestinal organs; microscopic structures such as glands, blood vessels, pits, villi, and crypts were also observed.     

First demonstration of a temporal coherence analysis through a parametric interferometer

We analyze the temporal coherence of an optical infrared radiation in the visible domain by using a Mach-Zehnder interferometer and a wavelength conversion stage in each arm. We exploit a sum frequency generation process in bulk PPLN crystal to convert the infrared radiation at 1.55 μm into 0.63 μm before the interferometric mixing. The applicability of the Wiener-Kintchine theorem through up-conversion processes is here demonstrated by direct comparisons among visible and infrared measurements.     

Influence of longitudinal spatial coherence on the signal of a scanning interferometer

The physical conditions for observing the longitudinal spatial coherence of light of an extended thermal source in interference experiments are defined. For experimental verification of these conditions a Michelson interferometer with a longitudinal scanning mirror was used. The influence of the longitudinal spatial coherence of thermal light on a fringe envelope is demonstrated experimentally.     

Elimination of dispersion effect in a white-light scanning interferometer by using a spectral analyzer

Optical Review - A general equation of the interference signal of white-light scanning interferometer (WSI) and its Fourier transform are derived. Based on these equations, a new method for...     

Changes in the spectral degree of coherence and spectral intensity of spatially and spectrally partially coherent cosh-Gaussian pulsed beams in free space

The analytical expressions for the spectral degree of coherence and spectral intensity of spatially and spectrally partially coherent cosh-Gaussian pulsed (ChGP) beams propagating in free space are derived. It is shown that the spectral degree of coherence and spectral intensity of spatially and spectrally partially coherent ChGP beams depend on the spatial correlation parameter, decentered parameter and temporal coherence length. Depending on the decentered parameter, the effective width of the spectral degree of coherence can be larger than, smaller than or equal to that of the spectral intensity. The results are illustrated by numerical examples.     

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.     

Optimization of Fizeau-based optical coherence tomography with a reference Michelson interferometer

The method of Fizeau-based optical coherence tomography with a reference Michelson interferometer has been optimized. The real-system noise and dynamic range have been estimated. The time parameters of the system are optimized according to the proposed noise model. A good correspondence between the numerical estimates and experimental data is shown.     

Detection of ultrafast phenomena by use of a modified Sagnac interferometer

We describe a time-division interferometer based on the Sagnac geometry for monitoring ultrafast changes in the real and the imaginary components of the refractive index as well as phase changes that are due to surface displacement. Particular advantages of this interferometer are its simple common-path design and operation at normal incidence with a microscope objective for both pumping and probing. Operation is demonstrated by detection of temperature changes and coherent phonon generation in a gold film.     

Shearing interferometer for quantifying the coherence of hard x-ray beams

We report a quantitative measurement of the full transverse coherence function of the 14.4 keV x-ray radiation produced by an undulator at the Swiss Light Source. An x-ray grating interferometer consisting of a beam splitter phase grating and an analyzer amplitude grating has been used to measure the degree of coherence as a function of the beam separation out to 30 microm. Importantly, the technique provides a model-free and spatially resolved measurement of the complex coherence function and is not restricted to high resolution detectors and small fields of view. The spatial characterization of the wave front has important applications in discovering localized defects in beam line optics.     

Scanning interferometer stabilized by use of Pancharatnam's phase

Time-resolved interferometric optical experiments demand both active stabilization of the relative phase between phase-locked pulses and continuous scanning of their time delay over a wide range. We describe a novel method that meets the above requirements in a convenient and simple way by use of the accumulated phase according to Pancharatnam [Proc.Indian Acad.Sci.Sect.A 44, 247 (1956)].     

Estimation of the degree of asphericity of a glass sphere using a vectorial shearing interferometer

The degree of asphericity is estimated by determining the average radius of curvature in different sections, at various points on the surface of a sphere, and the deviation from it. We employ the vectorial shearing interferometer (VSI) as the instrument to determine the radius of curvature from two subapertures of the transparent glass sphere. We incorporate the sphere as a thick lens into the interferometric setup, illuminating it with an expanded beam. The spherical aberration, introduced by the sphere in the wave front, depends on the local sphere radius, on the refraction index of the glass, and on the cone angle of the source. The wave front aberrated by the sphere impinges on the VSI. Here, the wave front is divided in two in amplitude, it is sheared vectorially, and it is superimposed with itself. The fringe pattern is formed in the intersection of the wave fronts. The shape of the resulting fringe pattern is directly related to spherical aberration. We estimate qualitatively the degree of asphericity, comparing the phase gradients in different sections of the sphere. Here, we report on the experimental setup to test the asphericity, the results with different vectorial shearing (magnitude and direction). Finally, we perform a comparison with the theoretical predictions.     

Correction of the second-order degree of coherence measurement

The measurement of the second-order degree of coherence [ge2TeτT] is one of the important methods used to study the dynamical evolution of photon-matter interaction systems. Here, we use a nitrogen-vacancy center in a diamond to compare the measurement of ge2TeτT with two methods. One is the prototype measurement process with a tunable delay. The other is a start-stop process based on the time-to-amplitude conversion(TAC) and multichannel analyzer(MCA) system, which is usually applied to achieve efficient measurements. The divergence in the measurement results is observed when the delay time is comparable with the mean interval time between two neighboring detected photons. Moreover, a correction function is presented to correct the results from the TAC-MCA system to the genuine ge2TeτT. Such a correction method will provide a way to study the dynamics in photonic systems for quantum information techniques.