Experiments in long-distance holographic imagery

Intensity correlation holography for remote phase sensing and 3D imaging

Holography is an established technique for measuring the wavefront of optical signals through interferometric combination with a reference wave. Conventionally the integration time of a hologram is limited by the interferometer coherence time, thus making it challenging to prepare holograms of remote objects, especially using weak illumination. Here, we circumvent this limitation by using intensity correlation interferometry. Although the exposure time of individual holograms must be shorter than the interferometer coherence time, we show that any number of randomly phase-shifted holograms can be combined into a single intensity-correlation hologram. In a proof-of-principle experiment, we use this technique to perform phase imaging and 3D reconstruction of an object at a ∼3 m distance using weak illumination and without active phase stabilization.

3D imaging through a highly heterogeneous double-composite random medium by common-path phase-shift digital holography

A method is proposed for 3D imaging through a highly heterogeneous double-composite random medium made of a thick mildly inhomogeneous medium followed by a thin strongly scattering layer. To realize the immunity to the heterogeneous random medium, a system of common-path phase-shift digital holography is designed in such a manner that the wavefront distortion caused by the first inhomogeneous medium is canceled out by the common-path geometry, and the influence of the random phase introduced by the second scattering layer is removed by the intensity-based recording of the digital hologram on the thin scattering layer. The validity of the method was confirmed by experiments.

Deep-turbulence wavefront sensing using digital holography in the on-axis phase shifting recording geometry with comparisons to the self-referencing interferometer

In this paper, we study the use of digital holography in the on-axis phase-shifting recording geometry for the purposes of deep-turbulence wavefront sensing. In particular, we develop closed-form expressions for the field-estimated Strehl ratio and signal-to-noise ratio for three separate phase-shifting strategies-the four-, three-, and two-step methods. These closed-form expressions compare favorably with our detailed wave-optics simulations, which propagate a point-source beacon through deep-turbulence conditions, model digital holography with noise, and calculate the Monte Carlo averages associated with increasing turbulence strengths and decreasing focal-plane array sampling. Overall, the results show the four-step method is the most efficient phase-shifting strategy and deep-turbulence conditions only degrade performance with respect to insufficient focal-plane array sampling and low signal-to-noise ratios. The results also show the strong reference beam from the local oscillator provided by digital holography greatly improves performance by tens of decibels when compared with the self-referencing interferometer.

Determining the phase and amplitude distortion of a wavefront using a plenoptic sensor

We have designed a plenoptic sensor to retrieve phase and amplitude changes resulting from a laser beam's propagation through atmospheric turbulence. Compared with the commonly restricted domain of (-π,π) in phase reconstruction by interferometers, the reconstructed phase obtained by the plenoptic sensors can be continuous up to a multiple of 2π. When compared with conventional Shack-Hartmann sensors, ambiguities caused by interference or low intensity, such as branch points and branch cuts, are less likely to happen and can be adaptively avoided by our reconstruction algorithm. In the design of our plenoptic sensor, we modified the fundamental structure of a light field camera into a mini Keplerian telescope array by accurately cascading the back focal plane of its object lens with a microlens array's front focal plane and matching the numerical aperture of both components. Unlike light field cameras designed for incoherent imaging purposes, our plenoptic sensor operates on the complex amplitude of the incident beam and distributes it into a matrix of images that are simpler and less subject to interference than a global image of the beam. Then, with the proposed reconstruction algorithms, the plenoptic sensor is able to reconstruct the wavefront and a phase screen at an appropriate depth in the field that causes the equivalent distortion on the beam. The reconstructed results can be used to guide adaptive optics systems in directing beam propagation through atmospheric turbulence. In this paper, we will show the theoretical analysis and experimental results obtained with the plenoptic sensor and its reconstruction algorithms.

Atmospheric turbulence correction using digital holographic detection: experimental results

The performance of long distance imaging systems is typically degraded by phase errors imparted by atmospheric turbulence. In this paper we apply coherent imaging methods to determine, and remove, these phase errors by digitally processing coherent recordings of the image data. In this manner we are able to remove the effects of atmospheric turbulence without needing a conventional adaptive optical system. Digital holographic detection is used to record the coherent, complex-valued, optical field for a series of atmospheric and object realizations. Correction of atmospheric phase errors is then based on maximizing an image sharpness metric to determine the aberrations present and correct the underlying image. Experimental results that demonstrate image recovery in the presence of turbulence are presented. Results obtained with severe turbulence that gives rise to anisoplanatism are also presented.

Deformed-helix ferroelectric liquid-crystal spatial light modulator that demonstrates high diffraction efficiency and 370-line pairs/mm resolution

New liquid-crystal media and photoconductor materials are being utilized in spatial light modulators to increase their resolution, diffraction efficiency, speed, and sensitivity. A prototypical device developed for real-time holography applications has shown an 8% diffraction efficiency from a holographic grating with a spatial frequency of 370 line pairs/mm (lp/mm). At 18 lp/mm the device has demonstrated a 31% diffraction efficiency with a 600-micros hologram write time using 400-nJ/cm(2) write beams.

Image transmission through a thick dynamic distorter by the photorefractive fanning effect

A new method of one-way image transmission through a thick dynamic distorter without the need for a reference beam or four-wave mixing is demonstrated. In this method there are only one object beam and one sampling beam. The response time of the photorefractive crystal must be much longer than the fluctuation period of the dynamic distorter. Thus the crystal responds only to the time-averaged intensity pattern of a rapidly varying object beam. The images, reconstructed with high fidelity, are picked up through the photorefractive fanning effect.

Time-multiplexed real-time one-way image compensation for high-spatial-frequency aberration correction

A new self-aligning geometry for real-time holographic image reconstruction for one-way imaging through a phase aberrator is demonstrated. The input beams are time multiplexed to isolate the diffracted image from the reference beams after the image beams propagate through the hologram. This geometry permits the image-bearing beam and the reference beams to copropagate through the holographic plane.

One-way imaging through an aberrator with spatially incoherent light by using an optically addressed spatial light modulator

A real-time holographic technique employing an optically addressed spatial light modulator was used to correct an incoherently illuminated image that had made a single pass through an aberrator. Aberrations that change slowly compared with the integration time of the spatial light modulator are fully corrected.

Effects of thick aberrators in one-way imaging schemes

The ability of one-way imaging schemes to correct for the effects of thick aberrators is examined. These schemes correct an image for the influence of an intervening aberrator in a single pass. Single-pass image correction is observed for thick aberrators that change in times that are short compared with the nonlinear response time of the four-wave mixing material. Image degradation is observed for static aberrators.

Achromatic volume holography using dispersive compensation for grating tilt

A method for achromatic volume holography is described using a combination of material and grating dispersion. With barium titanate as the dispersive material, a bandwidth of 250 nm is realized. The device may be used for broadband phase conjugation and image reconstruction.

Holographic correction of aberrations using PLZT

We describe the aberration correction capabilities of a holographic recording device made of antiferroelectricphase (7.6/70/30) ion-implanted lead lanthanum zirconate titanate. After a review of holographic aberration correction and a description of the device's holographic storage mode, we describe the experiment used to store a hologram which reduced approximately ten waves of aberration to a fifth of a wave. Star tests and interferograms demonstrate the accuracy of the correction.

Passive one-way aberration correction using four-wave mixing

We have demonstrated a passive method for recovering an optical image that has been degraded by being passed through a thin phase-aberrating medium. This method relies on a point source situated near the object of interest to sample the aberration impressed upon the wave front. Degenerate four-wave mixing in fluorescein-doped boric acid glass was used to reconstruct the wave front.

Image transmission through a turbulent medium using a point reflector and four-wave mixing. 2: Characteristics of the system

The characteristics of the one-way image transmission system presented in Part 1 are investigated in detail [Appl. Opt. 22, 2192 (1983)]. First, a general expression of the expectation of the transmitted image is derived for turbulence that may be typical in image transmission in the horizontal direction. Then, with the help of numerical examples, the image quality is discussed in terms of the point spread function for both thin layer and uniformly distributed turbulence. It is shown that the image transmission system is effective especially where turbulence exists relatively close to the transmission plane.

Space reflectors for radar and astronomy

A new concept to utilize large flat optical reflecting surfaces in space to increase by several orders of magnitude the sensitivity and resolution of earth laser radar and astronomy measurements is described. The physical principles on which simple structures can maintain the optical reflectance gratings in space are derived, and the data processing requirements of the measurements are discussed. Space and ground system designs are given for a high resolution earth resources laser radar sensor, a synchronous earth and planetary science laser radar system, and an astronomy observation system including a variable very long compound grating interferometer system.

Imaging through turbulent water using speckle reference holography

A dramatic reduction in image distortion resulting from turbulent water has been demonstrated using speckle reference holography. Conventional photographs taken simultaneously over the same path clearly show that the holographic technique eliminates most image distortion.

Long-range holography

Long-range holography is investigated for improving telescope resolution through a distorting atmosphere. Theoretical concepts are developed and operational parameters are discussed. A scaled simulation is made in the laboratory of a scene subtending an angle of 1 min of arc, illuminated by a laser and viewed holographically with a 40-cm telescope through fixed and moving atmospheres.

Temporally modulated holography

It is shown that temporal modulation of the object and/or reference wave in holographic interferometry can advantageously control fringe characteristics. The work presented is based on the concept that the hologram recording process acts as a filter applied to the cross correlation between the conjugate of the reference wave spectrum with the object wave spectrum. The filter is determined by the shuttering of the hologram. Specific types of modulation are considered both experimentally and theoretically. With single-sideband suppressed-carrier modulation of either the reference wave or object wave the hologram process can act as a heterodyne detection system for small-amplitude vibrations or as a system with a controllable number of fringes for large-amplitude vibrations. Certain types of modulation, such as sinusoidal phase modulation of the reference wave, are shown to give the relative phases of the object vibrations. Consideration is given to synthesizing general fringe patterns in multiple exposure holograms. Generalized strobe holography is also analyzed.

Coherent optical target recognition through a phase distorting medium

This paper presents a technique for distinguishing targets in a limited class, in case conventional imaging is unsuccessful due to a phase distorting medium closely overlaying the observation system. The procedure is to record the spatial density spectrum of a coherently illuminated object, construct its fourier transform, and then compare the resulting autocorrelation function with a limited number of possible autocorrelation functions. For a simple object, experimental results illustrate the effect that a close-lying phase distortion has on the conventional image as compared to that on the autocorrelation function.