The problem of imaging through thick scattering media is encountered in many disciplines of science, ranging from mesoscopic physics to astronomy. Photons become diffusive after propagating through a scattering medium with an optical thickness of over 10 times the scattering mean free path. As a result, no image but only noise-like patterns can be directly formed. We propose a hybrid neural network for computational imaging through such thick scattering media, demonstrating the reconstruction of image information from various targets hidden behind a white polystyrene slab of 3 mm in thickness or 13.4 times the scattering mean free path. We also demonstrate that the target image can be retrieved with acceptable quality from a very small fraction of its scattered pattern, suggesting that the speckle pattern produced in this way is highly redundant. This leads to a profound question of how the information of the target being encoded into the speckle is to be addressed in future studies.
Optical imaging through turbid media and around corner is a difficult challenge. Even a very thin layer of a turbid media, which randomly scatters the probe light, can appear opaque and hide any objects behind it. Despite many recent advances, no current method can image the object behind turbid media with single record using coherent laser illumination. Here we report a method that allows non-invasive single-shot optical imaging through turbid media and around corner via speckle correlation. Instead of being as an obstacle in forming diffractionlimited images, speckle actually can be a carrier that encodes sufficient information to imaging through visually opaque layers. Optical imaging through turbid media and around corner is experimentally demonstrated using traditional imaging system with the aid of iterative phase retrieval algorithm. Our method require neither scan of illumination nor two-arm interferometry or long-time exposure in acquisition, which has new implications in optical sensing through common obscurants such as fog, smoke and haze.
Phase space distributions such as the Wigner Distribution Function are very powerful tools to characterize the behavior of optical beams propagating in various optical systems. For instance, it is well known that when a beam passes through an ideal thin lens, the associated Wigner distribution function experiences a shear along the frequency axes. However, practical optical systems are not ideal. One must take the lens aberrations into account when analyzing and designing an optical system, in particular for imaging and metrology applications. In this paper, we present a theoretical and numerical study of how the Wigner Distribution Function evolves when a beam passes through a thin lens with aberrations characterized by the Zernike polynomials. The result shows that a deformation effect occurs in the Wigner distribution function when aberrations present. Thus the design of optical imaging and metrology systems is to reduce or eliminate the deformation from phase space optics point of view.
The conventional beam splitter is a must optical element for splitting one femtosecond laser pulse, however, it suffers from the unpleasant chromatic dispersion caused by passing through the material due to the broadband spectrum of femtosecond laser. Measurement of femtosecond laser is usually realized by using an autocorrelator, frequency-resolved optical grating (FROG), GRENOUILLE and spectral phase interferometry for direct electric-field reconstruction (SPIDER). Usually they have to use the beam splitter for generating the two identical pulses for measurement of femtosecond laser. In this paper, we report another approach by using reflective Dammann gratings as the beam splitter. Two-layered reflective Dammann gratings provide an alternative solution of splitting by using the reflective structure, which has nice features of easy alignment, no chromatic dispersion, simple structure and low cost. Experimental implementation of the Dammann FROG apparatus by using two-layered reflective Dammann gratings generates the almost same result as the FROG method. This structure should be highly interesting as a novel approach for the measurement of femtosecond laser pulses. In addition, this structure has the possibility to replace the beam splitter in various applications, such as the pump-probe technique in femtochmistry and semiconductor, etc..
A Dammann grating can generate an array of uniform intensity and equally spaced spots for an incoming monochromatic light beam. But chromatic dispersion will occur when a beam of femtosecond laser pulse, which contains a broad spectral bandwidth, is split by a Dammann grating. Furthermore, the quantity of chromatic dispersion is different in each diffraction order. As a result, diffraction spots of splitting beams are becoming more elliptical as the diffraction order increases. In this paper, we propose a method of using an m time density’s grating to collimate the mth order beam that is split by a Dammann grating. In this way, an array of femtosecond laser beams that are eliminated lateral chromatic dispersion can be obtained by using a Dammann grating and a group of compensation gratings. At the same time, the increased width of the compensated output pulse is briefly discussed with Kirchhoff-Fresnel integral, and in the case of the pulse duration of 100fs, the increased amount of the pulse width at the different diffraction orders and the shape variation of the output diffraction spots are not serious. As a new kind of beam splitter, this splitting and compensation system is high efficiency and material dispersion is avoided if reflection gratings are employed. It should be highly interesting in practical applications of splitting femtosecond laser pulses for pulse-width measurement, pump-probe measurement, and micromachining at multiple points.