This PDF file contains the front matter associated with SPIE Proceedings Volume 7924, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Partially coherent beams, such as Gaussian Schell-model beam, partially coherent dark hollow beam, partially coherent flat-topped beam and electromagnetic Gaussian Schell-model beam, have important applications in free space optical communications, optical imaging, optical trapping, inertial confinement fusion and nonlinear optics. In this paper, experimental generations of various partially coherent beams are introduced. Furthermore, with the help of a tensor method, analytical formulae for such beams propagating in turbulent atmosphere are derived, and the propagation properties of such beams in turbulent atmosphere are reviewed.

The scintillation properties of Airy beam arrays in atmospheric turbulence are investigated. Similar to their propagation in free space, the average propagation paths of Airy beams are also parabolic in turbulence. By utilizing this self-bending property, the constituent Airy beamlets propagate through relatively independent regions of turbulence and fully overlap at the on-axis detector. Through numeric simulations, it is shown that the scintillation of Airy beam array is significantly reduced compared to a single Airy beam.

We introduce the concept of pseudo-Bessel correlated beams and investigate their scintillation properties on propagation through turbulence. In weak turbulence the scintillation index of pseudo-Bessel correlated beams is formulated by using the Rytov approximation. The study of scintillation is extended to strong turbulence by numeric simulations. It is shown that by choosing an appropriate coherence parameter, pseudo-Bessel correlated beams have lower scintillation than comparable fully coherent beams in both weak and strong turbulence.

In our report, a method of evaluation of the mutual coherence function (MCF) of optical wave propagating in turbulent atmosphere is proposed. The method is based on using inverse Fresnel transform and 2-D Fast Fourier Transformation procedure (FFT) and provides a high accuracy in various propagation conditions from weak up to strong optical turbulence regime. This technique allows significantly reducing the evaluation time of MCF. The proposed method is suitable for optical waves with arbitrary initial distribution of amplitude and phase. Results of investigations on the degradation of coherence of symmetric dark hollow beams (DHB) propagating in turbulent atmosphere are presented. Analysis of evolution of MCF is brought up, and some characteristics of DHB, such as mean intensity distribution, and moments of Wigner distribution are calculated for various kinds of profiles of structural characteristic of refractive index. All of the evaluations for DHB are fulfilled for Kolmogorov spectrum of correlation function of refractive index fluctuations. The comparison between mean intensity calculations within the proposed method and method based on semi-analytical approach using of quadratic approximation of spherical wave structure function, is presented.

We report on the analysis of experimental data collected at the United States Naval Academy in the Summer and Fall of 2010. A low-power red He-Ne laser source was used to generate a Gaussian beam. The beam was directed horizontally above the ground and over the water for 400 m during daytime. The transverse cross-section of the beam was projected on a white screen and imaged using a ccd camera. The histogram of the fluctuating intensity at the center of the beam was obtained from the sequence of photographs. Comparison was made of the histogram and probability density functions of fluctuating intensity based on two existing analytic models, Gamma-Gamma and Gamma-Laguerre. Also the comparison of the beam statistics above the ground and over the water was performed.

The influence of Earth atmospheric turbulence on the propagation of a picosecond laser pulse has been investigated from point of view detection with high temporal resolution. The results have been interpreted for optical time scale synchronization link allowing picosecond precision and accuracy in ground-to-space time transfer on a single photon signal levels. The details in laser beam position changes, phase wave-front deformation or beam profile changes were not studied like in adaptive optics as the goal of time transfer link is not the imaging but time tagging. The figure of merit of presented results is the time of propagation, its absolute delay and jitter. The correlation of the atmospheric turbulence with the propagation delay fluctuation was measured. The physical reason of the fluctuation of propagation time of laser pulse on picosecond level is the same, but the entirely different approach in comparison to adaptive optics was used to describe the effect.

Advances in the fields of optics and optical communications have created a demand for effectively measuring relative phase changes along an optical path or within an optical system. We present a method for obtaining these measurements using an interferometric setup with processing involving Empirical Mode Decomposition and the Hilbert Transform. In this work, the Hilbert Transform algorithm is justified by accurately measuring the phase changes in software generated signals. Progress and improvements are shown regarding the ongoing design and implementation of an experimental benchtop setup. This testbed will prove the method in applications such as measuring and recording phase changes caused by propagating light through a turbulent freespace channel.

The growing need for high data-rate communication both through the atmosphere and the ocean (sub-sea) has stimulated considerable interest in optical wireless communication (OWC) technologies. The main advantages of OWC as compared with RF communication in the atmosphere and with acoustic communication in sub-sea applications are a) high achievable data-rate, b) small size of equipment and c) low power-consumption. On the other hand the characteristics of the communication channel in both scenarios are stochastic with high values of variance, which severely degrades OWC communication system performance. In this paper we present a tutorial discussing the effects of random media on OWC and expand on two examples: Monte-Carlo simulation for sub-sea communication and mathematical synthesis using Meijer G-function for OWC through atmospheric turbulence. These two examples demonstrate that it is possible to gain significant insights on the effects of the random channel on system performance. The results of the different analysis methods could also indicate solutions for the improvement of performance using adaptive solutions or for extending the communication range by applying a multi-hop concept. We summarize the paper with a brief review of two emerging research fields that could, surprisingly, benefit from the characteristics of light propagation through random media and its effect on the communication system performance. The first research field is trans-cutaneous OWC and the second is an unguided optical communication bus for next-generation computers.

A new wavefront control approach for mitigation of atmospheric turbulence-induced wavefront phase aberrations in coherent fiber-array-based laser beam projection systems is introduced and analyzed. This approach is based on integration of wavefront sensing capabilities directly into the fiber-array transmitter aperture. In the coherent fiber array considered, we assume that each fiber collimator (subaperture) of the array is capable of precompensation of local (onsubaperture) wavefront phase tip and tilt aberrations using controllable rapid displacement of the tip of the delivery fiber at the collimating lens focal plane. In the technique proposed, this tip and tilt phase aberration control is based on maximization of the optical power received through the same fiber collimator using the stochastic parallel gradient descent (SPGD) technique. The coordinates of the fiber tip after the local tip and tilt aberrations are mitigated correspond to the coordinates of the focal-spot centroid of the optical wave backscattered off the target. Similar to a conventional Shack-Hartmann wavefront sensor, phase function over the entire fiber-array aperture can then be retrieved using the coordinates obtained. The piston phases that are required for coherent combining (phase locking) of the outgoing beams at the target plane can be further calculated from the reconstructed wavefront phase. Results of analysis and numerical simulations are presented. Performance of adaptive precompensation of phase aberrations in this laser beam projection system type is compared for various system configurations characterized by the number of fiber collimators and atmospheric turbulence conditions. The wavefront control concept presented can be effectively applied for long-range laser beam projection scenarios for which the time delay related with the double-pass laser beam propagation to the target and back is compared or even exceeds the characteristic time of the atmospheric turbulence change - scenarios when conventional target-in-the-loop phase-locking techniques fail.

We numerically examine the spatial evolution of the structure of coherent and partially coherent laser beams (PCBs), including the optical vortices, propagating in turbulent atmospheres. The influence of beam fragmentation and wandering relative to the axis of propagation (z-axis) on the value of the scintillation index (SI) of the signal at the detector is analyzed. A method for significantly reducing the SI, by averaging the signal at the detector over a set of PCBs, is described. This novel method is to generate the PCBs by combining two laser beams - Gaussian and vortex beams, with different frequencies (the difference between these two frequencies being significantly smaller than the frequencies themselves). In this case, the SI is effectively suppressed without any high-frequency modulators.

Free-space optical (FSO) communication systems suffer from average power loss and instantaneous power fading due to the atmospheric turbulence. The channel fading probability density function (pdf) is of critical importance for FSO communication system design and evaluation. The performance and reliability of FSO communication systems can be greatly enhanced if fast-tacking devices are employed at the transmitter in order to compensate laser beam wander at the receiver aperture. The fast-tracking method is especially effective when communication distance is long. This paper studies the fading probability density functions of both fast-tracked and untracked FSO communication channels. Large-scale wave-optics simulations are conducted for both tracked and untracked lasers. In the simulations, the Kolmogorov spectrum is adopted, and it is assumed that the outer scale is infinitely large and the inner scale is negligibly small. The fading pdfs of both fast-tracked and untracked FSO channels are obtained from the simulations. Results show that the fast-tracked channel fading can be accurately modeled as gamma-distributed if receiver aperture size is smaller than the coherence radius. An analytical method is given for calculating the untracked fading pdfs of both point-like and finite-size receiver apertures from the fast-tracked fading pdf. For point-like apertures, the analytical method gives pdfs close to the well-known gamma-gamma pdfs if off-axis effects are omitted in the formulation. When off-axis effects are taken into consideration, the untracked pdfs obtained using the analytical method fit the simulation pdfs better than gamma-gamma distributions for point-like apertures, and closely fit the simulation pdfs for finite-size apertures where gamma-gamma pdfs deviate from those of the simulations significantly.

We present results on the design, development and initial testing of a fiber-optic based RF-modulated lidar transmitter operating at 532nm, for underwater imaging application in littoral waters. The design implementation is based on using state-of-the-art high-speed FPGAs, thereby producing optical waveforms with arbitrary digital-RF-modulated pulse patterns with carrier frequencies ≥ 3GHz, with a repetition rate of 0.5-1MHz, and with average powers ≥5W (at 532nm). Use of RF-modulated bursts above 500MHz, instead of single optical pulse lidar detection, reduces the effect of volumetric backscatter for underwater imaging application, leading to an improved signal-to-noise-ratio (SNR) and contrast, for a given range. Initial underwater target detection tests conducted at Patuxent River Naval Air Station, MD, in a large water-tank facility, validates the advantages of this hybrid-lidar-radar (HLR) approach for improved underwater imaging, over a wide range of turbidity levels and both white and black targets. The compact, robust and power-efficient fiber laser architecture lends very well to lidar sensor integration on unmanned-underwater-vehicle (UUV) platforms. HLR transmitters can also provide similar advantages in active-sensing situations dominated by continuous backscatter, e.g. underwater communications, imaging through smoke and fire environment, rotor-craft landing in degraded visual environment, and pointing-tracking of active-EO sensors through fog.

An open-path Tunable Diode Laser Absorption Spectroscopy (TDLAS) system composed of narrow band (~300 kHz) diodes fiber coupled to a 12" Ritchey-Chrétien transmit telescope has been developed to study atmospheric transmission of key High Energy Laser wavelengths. The ruggedized system has been field deployed and tested for propagation distances of greater than 1 km. Initial experiments were performed in the vicinity of molecular oxygen X³Σ-_g to b¹Σ+_gelectronic transition lines near 760 nm. The potassium version of the Diode Pumped Alkali Laser (DPAL) operates in between two of the sharp oxygen rotational features in the PP and the PQ branches. By scanning across many laser free spectral ranges and monitoring the laser frequency with a very precise wavemeter, the full structure of the oxygen molecular feature is observed. The device can also be used to observe rotational temperatures, oxygen concentrations, and total atmospheric pressure.

We report on a thulium doped silica fiber ASE source for absorption spectroscopy of CO₂. The average spectral power of this source was 2.3-6.1 μW/nm. This low spectral power of this source posed limitation in the sensitivity of the system which was overcome by using an ultrashort pulsed Raman amplifier system with 50-125 μW/nm average spectral power. This system produced CO₂ sensitivity better than 300 ppm making measurement of CO₂ possible at standard atmospheric concentrations.

On the basis of the angular spectrum representation of stochastic, statistically stationary scalar fields, the Rytov's perturbation technique for propagation in weakly fluctuating media and the first Born approximation for weak scattering, we develop a technique for transmission of stochastic fields through turbulence containing randomly distributed particles. Results for transmission of the deterministic (laser) field may be obtained from our general results as a limiting case. We show how our technique can be applied specifically for the atmospheric turbulence, but in general can also be of interest for propagation in oceanic turbulence and biological tissues.