Ghost imaging and ghost diffraction with pseudo-thermal light generated by means of a programmable SLM

Optical phase microscopy provides a view of objects that have minimal to no effect on the detected intensity of light that are unobservable by standard microscopy techniques. Since its inception just over 60 years ago that gave us a vision to an unseen world and earned Frits Zernike the Nobel prize in physics in 1953, phase microscopy has evolved to find various applications in biological cell imaging, crystallography, semiconductor failure analysis, and more. Two common and commercially available techniques are phase contrast and differential interference contrast (DIC). In phase contrast method, a large portion of the unscattered light that accounts for the majority of the light passing unaffected through a transparent medium is blocked to allow the scattered light due to the object to be observed with higher contrast. DIC is a self-referenced interferometer that transduces phase variation to intensity variation. While being established as fundamental tools in many scientific and engineering disciplines, the traditional implementation of these techniques lacks the ability to provide the means for quantitative and repeatable measurement without an extensive and cumbersome calibration. The rapidly growing fields in modern biology meteorology and nano-technology have emphasized the demand for a more robust and convenient quantitative phase microscopy. The recent emergence of modern optical devices such as high resolution programmable spatial light modulators (SLM) has enabled a multitude of research activities over the past decade to reinvent phase microscopy in unconventional ways. This work is concerned with an implementation of a DIC microscope containing a 4-f system at its core with a programmable SLM placed at the frequency plane of the imaging system that allows for employing Fourier pair transforms for wavefront manipulation. This configuration of microscope provides a convenient way to perform both wavefront shearing with quantifiable arbitrary shear amount and direction as well as phase stepping interferometry by programming the SLM with a series of numerically generated patterns and digitally capturing interferograms for each step which are then used to calculate the objects phase gradient map. Wavefront shearing is performed by generating a pattern for the SLM where two phase ramp patterns with opposite slopes are interleaved through a random selection process with uniform distribution in order to mimic the simultaneous presence of the ramps on the same plane. The theoretical treatment accompanied by simulations and experimental results and discussion are presented in this work.

We demonstrate a novel ghost imaging (GI) scheme based on one single-photon-counting detector with subsequent photon statistics analysis. The key idea is that instead of measuring correlations between the object and reference beams such as in standard GI schemes, the light of the two beams is superimposed. The photon statistics analysis of this mixed light allows us to determine the photon number distribution as well as to calculate the central second-order correlation coefficient. The image information is obtained as a function of the spatial resolution of the reference beam. The performance of this photon-statistics-based GI system with one single detector (PS-GI) is investigated in terms of visibility and resolution. Finally, the knowledge of the complete photon statistics allows easy access to higher correlation coefficients such that we are able to perform here third- and fourth-order GI. The PS-GI concept can be seen as a complement to already existing GI technologies thus enabling a broader dissemination of GI as a superior metrology technique, paving the road for new applications in particular with advanced photon counting detectors.

We study the influence rules of the speckle size of a light source on ghost imaging, and propose a type of speckle pattern to improve the quality of ghost imaging. The results show that image quality will first increase and then decrease with the increase in speckle size, and there is an optimal speckle size for a specific object. At the same time, by using a random distribution of speckle positions, a type of displacement speckle pattern is designed, and the imaging quality is better than that of random speckle patterns. These results are of great significance for finding the best speckle patterns suitable for detecting targets, which further promotes practical applications of ghost imaging.

In an imaging system, resolution and signal-to-noise ratio (SNR) are two important indexes to characterize imaging quality. Ghost imaging is a novel imaging method whose imaging resolution and SNR are affected by the speckle size. In this paper, the relation between speckle size and resolution as well as that between speckle size and SNR in the GI system is analyzed in detail. It is shown that the critical resolution, resolvable minimum-separation between two adjacent objects, is approximately equal to the speckle size (speckle diameter). There exists an optimum SNR when the speckle size is larger than the object size. Based on our conclusion, we propose a scheme to enhance the critical resolution of the GI system by using a vortex beam, and the enhancement ability under different topological charges is clearly presented, which can be quantized by a simple formula.

Near field and far field correlations of thermal light

Along with the rapid growth in fixed diffractive and holographic element technology in recent years has come intensive interest in active, dynamic diffractive elements which can be electrically programmed or switched. Two different classes of electrically controllable diffractive structures may be distinguished. The more familiar class are the programmable spatial light modulators, addressed on a pixel-by-pixel basis. Less familiar is the emerging concept of monolithically switched holograms, now an active subject in several laboratories. These have a fixed, prefabricated diffractive structure whose diffraction efficiency can be modulated as a whole. Whereas spatial light modulators typically are restricted to relatively small arrays of pixels on the order of 256X256 and correspondingly low diffraction efficiencies, monolithic holograms can be of extremely high resolution, optical quality, and diffraction efficiency, with the equivalent of one million times higher pixel density. Such elements can yield new devices significantly different from SLMs, particularly if the material is also of sufficiently high optical quality to permit series stacking. Along these lines, Foster-Miller has been developing applications for a promising material, liquid crystal infused Polaroid holographic photopolymer, which appears to be a possible basis for many such devices. The present paper reviews some of our applications trials to date, including lab demonstrations of laser beam diverters, programmable beamlet array generators, dynamic lenses, fiber optic switches, both reconfigurable and continuously tunable wavelength filters, holographic optical memories and memory structures, and thermal applications for switching of white light or sunlight.

It is difficult to obtain a clear image in underwater turbulence environment with classical imaging methods due to the absorption, scattering, and underwater turbulence on the propagation beam. However, ghost imaging (GI), a non-locally imaging technique, has shown the turbulence-free ability in atmospheric turbulence by exploiting the second-order correlation between the signal beam and the reference beam. In this paper, we experimentally investigate the imaging quality of GI affected by the underwater environment, where the underwater environment is simulated by a 1 m × 0.4 m × 0.4 m tank with distilled water. The water temperature is controlled by a heater inside the tank, and a temperature gradient is obtained by putting the heater at different positions of the tank. The water vibration is produced by a heavy force, and the turbid medium is obtained by dissolving very small specks of CaCO3 in the water. A set of Hadamard speckle pattern pairs are generated and modulated on the incident beam, and then the beam illuminates on an unknown object after passing through the simulated underwater environment. With the second-order correlations, the image is reconstructed under different temperature gradients, water vibration, and turbid medium ratios. The results show that GI has the turbulence-free ability under lower temperature gradient, water vibration, and turbid media. The structural similarity image measurement (SSIM) values of the reconstructed images only start to decrease when the temperature gradient is greater than 4.0 °C. The same temperature gradient produced at the different positions has a little effect on the quality of the underwater GI.

Alternative use of reflection spatial light modulator in phase shifting digital Gabor holography

Optical imaging in scattering media and its applications are challenging and meaningful. In this paper, we propose and experimentally verify a new optical authentication method using structured-detection-based ghost imaging (GI) in scattering media. Object wave is disturbed by multiple diffusers, and then sequentially modulated by a series of random amplitude-only patterns embedded in a spatial light modulator (SLM). The modulated wave passes through another scattering medium, and its intensity is measured by using a single-pixel bucket detector without spatial resolution. During the decryption and authentication, a reference pattern is first retrieved by using all recorded single-pixel intensity signals. Subsequently, a small number of the recorded single-pixel intensity signals are further randomly selected, and a 1-bit compression operation is applied to these selected intensity signals to generate binary signals as ciphertext. The random amplitude-only patterns corresponding to the selected single-pixel intensity signals serve as principal security keys, and wavelength, axial distance and pixel size can serve as supplementary keys. Two strategies are further developed for the decryption and authentication. It is experimentally verified that the proposed method possesses high robustness and high discrimination capability. The proposed method established by using scattering media can significantly enrich optical security, and provides a promising approach for optical authentication.

In ghost imaging, an illumination light is split into test and reference beams which pass through two different optical systems respectively and an image is constructed with the second-order correlation between the two light beams. Since both light beams are diffracted when passing through the optical systems, the spatial resolution of ghost imaging is in general lower than that of a corresponding conventional imaging system. When Gaussian-shaped light spots are used to illuminate an object, randomly scanning across the object plane, in the ghost imaging scheme, we show that by localizing central positions of the spots of the reference light beam, the resolution can be increased by a factor of same as that of the corresponding conventional imaging system. We also find that the resolution can be further enhanced by setting an appropriate threshold to the bucket measurement of ghost imaging.

Phase modulating spatial light modulators (SLMs) can be used to alter the shape of a laser wavefront to achieve a deflection or change in the shape of a laser beam. This paper reports the results of characterization, simulation and optimization of a one-dimensional liquid crystal (LC) SLM. The device has a large ratio between LC layer thickness and pixel pitch that results in a fringing field between pixels. In effect, the applied phase patterns will be lowpass filtered and the loss of high frequency components limits, for instance, the usable steering range. A method is presented where intensity measurements in the far field are used to determine how the phase modulation at the SLM is distorted. The inhomogeneous optical anisotropy of the device was determined by modelling the liquid crystal director distribution within the electrode-pixel structure. Finite-difference time-domain (FDTD) simulations were used to calculate the light propagation through the LC. The simulated phase distortion was compared with the experimental results. A voltage compensation scheme to improve the diffraction efficiency was developed utilizing the measured and simulated results. It is demonstrated that a modification of the voltage patterns can give a better realization of high frequency components in the phase distribution and an increase in maximum steering angle by a factor two.

Liquid crystal television (LCTV) continues to play a useful role as a spatial light modulator in the development and evaluation of systems for optical image processing. We outline new addressing electronics developed for a commercially available LCTV that permit writing to individual pixels at an improved display up-date rate and allow the input video signal to cover a much greater transmittance range of the TV display for black and white pixels. We illustrate this by measuring the diffraction efficiency for gratings written onto the display. For vertical gratings written along the display columns the diffraction efficiency is increased significantly, but there is no improvement for horizontal gratings. Some merits of the modified LCTV modulator for optical processing applications are considered briefly.

The application of a programmable magneto-optic spatial light modulator to white-light optical signal processing is presented. We have shown that the magneto-optic device responds to the polarized white-light, in which a wide range of color object patterns can be generated. Since the magneto-optic device is a transmitted type spatial light modulator, it is very suitable for real-time programmable spatial filter synthesis and object pattern generation for optical signal processings. Experimental demonstrations of some of the elementary spatial filter syntheses and pseudocolor encodings are provided.

In contrast to the mechanical scanning procedure described in the standard ISO/DIS 11146, the use of electronically tunable focal length lenses has proved its capability for the measurement of the laser beam propagation factor (M2) without moving components. Here, we demonstrate a novel experimental implementation where we use a low-cost programmable liquid crystal spatial light modulator (SLM) for sequentially codifying a set of lenses with different focal lengths. The use of this kind of modulators introduces some benefits such as the possibility for high numerical aperture or local beam control of the phase of the lenses which allows for minimizing systematic errors originated by lens aberrations. The beamwidth, according to the second-order moment of the irradiance, is determined for each focal length by using a digital sensor at a fixed position with respect to the spatial light modulator. After fitting the measured data to the theoretical focusing behavior of a real laser beam, the beam propagation factor is obtained. We successfully validated the results in the laboratory where a full digital control of the measurement procedure without mechanical scanning was demonstrated.

In this letter, we report on the development and characterization of an InGaAsP semiconductor optical amplifier-(SOA) based ring cavity laser that emits up to 16 wavelengths in the 1550-nm region. The laser is loss modulation mode locked at up to 10 GHz with temporal pulses of /spl sim/25 ps. Spectral control of the emission is accomplished by an intracavity 4-f spectral pulse shaper with a programmable liquid crystal spatial light modulator (SLM) in the Fourier plane. Voltage control of the transmissibility of the pixels of the SLM defines the wavelength channels and allows flattening of channels to produce a uniform intensity of channel outputs.