Rotating Ground Glass Diffuser Research Articles

Generation of hyper-bunched light by single Gaussian and non-Gaussian scattering processes

We derive theoretically that hyper-bunched light with a central normalized second-order correlation coefficient of six can be realized by a single Gaussian scattering process of parametric down conversion (PDC) light with a central normalized second-order correlation coefficient of three. The Gaussian scattering process is realized by a rotating ground-glass diffuser. We show that the photon counting probability distribution in this case obeys a Tricomi confluent hypergeometric function U[1+n,3/2,1/⟨n⟩] dependence. Furthermore, we also study non-Gaussian light-scattering probabilities that together with the different impinging light statistics give rise to new photon statistics accompanied by a variety of new values of the second-order correlation coefficient of the scattered light. These theoretical calculations suggest experiments using twin photons from a PDC process and characterizing their photon statistics properties before and after the scattering at the rotating diffuser. These investigations contribute to a more comprehensive understanding of the scattering process, the generated light, and new applications.

Single-Pixel Diffuser Camera

We present a compact, diffuser-assisted, single-pixel computational camera. A rotating ground glass diffuser is adopted, in preference to a commonly used digital micro-mirror device (DMD), to encode a two-dimensional (2D) image into single-pixel signals. We retrieve images with an 8.8% sampling ratio after the calibration of the pseudo-random pattern of the diffuser under incoherent illumination. Furthermore, we demonstrate hyperspectral imaging with line array detection by adding a diffraction grating. The implementation results in a cost-effective single-pixel camera for high-dimensional imaging, with potential for imaging in non-visible wavebands.

Improving the intensity-contrast image of a noisy digital hologram by convolution of Chebyshev type 2 and elliptic filters.

In this paper, a new, to the best of our knowledge, technique convolves the windowed Fourier filtering (WFF) of the Fresnel transform with the transfer functions of both Chebyshev type 2 and elliptic filters to enhance the intensity-contrast image of a noisy digital hologram. The recorded digital hologram is reconstructed by the Fresnel approach, the reconstructed intensity-contrast image is transformed by WFF, and the obtained spectrum is convolved in frequency domain with the transfer functions of Chebyshev type 2 and elliptic filters. The result of convolution is transformed by inverse WFF to produce a speckle-free image with a sharp roll-off and no ripples in both pass- and stop-bands. The experimental results with a die in the presence and absence of a rotating ground glass diffuser are shown and demonstrate that the resolution can be effectively enhanced with simple setup and procedure. The proposed technique can improve the capabilities of digital holography in three-dimensional (3D) microscopy.

Optical anisotropy measurement in normal and cancerous tissues: backscattering technique.

Investigating the deformation of tissue architecture is one of the most important clinical methods for cancer diagnosis. Optical methods are now widely developed for rapid, precise, and real-time assessment of these alterations at the microscopic scale. One of the proposed methods is enhanced backscattering (EBS) technique that allows in-vivo measurement of the optical scattering characteristics. Here, EBS technique is employed to evaluate the optical anisotropy of human epithelial tissues as a measure to distinguish between normal and cancerous one. Orientation dependence of the mean scattering length is assessed in healthy and cancerous tissues of five different human organs i. e. uterus, bladder, colon, kidney, and liver. Helicity preserving channel and rotating ground glass diffuser are utilized to eliminate the polarization induced anisotropy and the background speckle noises respectively. Analysis of the backscattering cones recorded by a high-resolution CCD camera reveals the modification of the strength and degree of optical anisotropy in different tissues during cancer progression. Pathology data affirm the correlation between the experimental results and the morphological alteration of the epithelial cells in each carcinoma type. In general, tissues with fibrous constructional cells are subject to a decrease in anisotropy due to cancer, whereas those with cuboidal cells experience an increase in anisotropy. This complementary information enhances the potency of the EBS technique as a fast, non-destructive, and easily accessible tool for real-time tissue diagnosis.

Characterization of the electromagnetic Gaussian Schell-model beam using first-order interference.

We propose a method for the characterization of electromagnetic Gaussian Schell-model (EMGSM) beams. This method utilizes the first-order interference consisting of polarization-state projections along with the two-point (generalized) Stokes parameters. The second-order field correlations employed in this method enable us to determine both the magnitude and the argument of the complex degree of electromagnetic coherence. We experimentally demonstrate this method by characterizing an EMGSM beam, which is synthesized using a laser beam passing through a rotating ground glass diffuser. This beam-characterization method is expected to be potentially useful for probing the partially coherent and partially polarized beams, and have tremendous applications in broad areas of optical communication and beam propagation.

Development and characterization of a source having tunable partial spatial coherence and polarization features

Partially coherent fields offer more robustness to losses occurring due to atmospheric propagation, and are, therefore, considered a promising candidate for free-space optical communication (both classical and quantum). The quantification of loss immunity for such fields requires a versatile source having tunable partial coherence and polarization attributes. In this work, we report construction of an optical source having tunable polarization and spatial coherence using an interferometric arrangement along with a variable beam expender. Realization of such source first requires a thorough study of spatial coherence properties of the laser beam passing through a rotating ground glass diffuser for different sizes of the source at the diffuser plane. It is found that spatial coherence of laser beam decreases, as size of the source increases. We also investigate the effect of rotation speed of diffuser on spatial coherence of a laser beam and find that it does not change significantly with rotation speed of the diffuser. This study also validates the van Cittert-Zernike theorem for our multi-scatterer source having linear, circular and spatial polarizations confirming enhancement of coherence with free-space propagation.

Step-by-step guide to reduce spatial coherence of laser light using a rotating ground glass diffuser.

Wide field-of-view imaging of fast processes in a microscope requires high light intensities motivating the use of lasers as light sources. However, due to their long spatial coherence length, lasers are inappropriate for such applications, as they produce coherent noise and parasitic reflections, such as speckle, degrading image quality. Therefore, we provide a step-by-step guide for constructing a speckle-free and high-contrast laser illumination setup using a rotating ground glass diffuser driven by a stepper motor. The setup is easy to build, cheap, and allows a significant light throughput of 48%, which is 40% higher in comparison to a single lens collector commonly used in reported setups. This is achieved by using only one objective to collect the scattered light from the ground glass diffuser. We validate our setup in terms of image quality, speckle contrast, motor-induced vibrations, and light throughput. To highlight the latter, we record Brownian motion of micro-particles using a 100× oil immersion objective and a high-speed camera operating at 2000 Hz with a laser output power of only 22 mW. Moreover, by reducing the objective magnification to 50×, sampling rates up to 10,000 Hz are realized. To help readers with basic or advanced optics knowledge realize this setup, we provide a full component list, 3D-printing CAD files, setup protocol, and the code for running the stepper motor.

Speckle contrast reduction of laser light using a chiral nematic liquid crystal diffuser

High coherence in laser light causes spatially distributed interference called speckle. In applications such as holographic projection, this undesirable side effect degrades image clarity. The current methods of speckle reduction, such as a rotating ground-glass diffuser, require additional bulky moving parts. Here, we present an alternative technology based upon a compact, electrohydrodynamic chiral nematic liquid crystal device. A spatially random phase modulation of the incident light is achieved through the electrohydrodynamic instabilities that are induced by an alternating electric field. Using a chiral nematic liquid crystal device that is doped with an ionic compound, we find that the speckle contrast can be reduced by as much as 80%.

Holographic imaging through a scattering medium by diffuser-aided statistical averaging.

We introduce a practical digital holographic method capable of imaging through a diffusive or scattering medium. The method relies on statistical averaging from a rotating ground glass diffuser to negate the adverse effects caused by speckle introduced by a static diffuser or scattering medium. In particular, a setup based on Fourier transform holography is used to show that an image can be recovered after scattering by introducing an additional diffuser in the optical setup. This method is capable of recovering object information from behind a scattering layer in biomedical or military imaging applications.

Comment on “Observation of anticorrelation in incoherent thermal light fields”

Recently, Chen et al. [Phys. Rev. A 84, 033835 (2011)] reported observation of anticorrelated photon coincidences in a Mach-Zehnder interferometer whose input light came from a mode-locked Ti:sapphire laser that had been rendered spatially incoherent by passage through a rotating ground-glass diffuser. They provided a quantum-mechanical explanation of their results, which ascribes the anticorrelation to two-photon interference. They also developed a classical-light treatment of the experiment and showed that it was incapable of explaining the anticorrelation behavior. Here we show that semiclassical photodetection theory, i.e., classical electromagnetic fields plus photodetector shot noise, does indeed explain the anticorrelation found by Chen et al. The key to our analysis is properly accounting for the disparate time scales associated with the laser's pulse duration, the speckle-correlation time, the interferometer's differential delay, and the duration of the photon-coincidence gate. Our result is consistent with the long-accepted dictum that laser light which has undergone linear-optical transformations is classical-state light, so that the quantum and semiclassical theories of photodetection yield quantitatively identical results for its measurement statistics. The interpretation provided by Chen et al. for their observations implicitly contradicts that dictum.

Wide-field optically sectioning fluorescence microscopy with laser illumination.

We describe an extremely simple method by which optically sectioned fluorescence images may be obtained with conventional microscopes using laser illumination. A one-dimensional grid pattern is introduced into the illumination system, together with a rotating ground glass diffuser. This causes an image of the grid pattern to be projected into the specimen. Images taken at three spatial positions of the grid are processed in a simple manner to provide optically sectioned images of fluorescent specimens.