Comparison of maritime measurements of Cn2 with NAVSLaM model predictions.

In this paper, a study is made of the refractive index structure parameter Cn2, as derived from angle-of-arrival (AOA) measurements made on the beam after propagation along a 16 km slant path across the Chesapeake Bay. These measurements are compared with Cn2 estimates derived from the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM), which are based upon prevailing meteorological conditions. Correlation coefficients for the reported data vary between 0.64 and 0.9. Despite the Chesapeake Bay theoretically being a difficult location for employing a Monin-Obukhov similarity theory-based model such as NAVSLaM, the agreement between the AOA Cn2 measurements and the NAVSLaM Cn2 estimates was, in many cases, good. A possible explanation of this agreement between the modeled and measured Cn2 values is that the large air-water temperature differences encountered provided such strong forcing for the NAVSLaM model that any potential violations of the Monin-Obukhov similarity theory assumptions had only a secondary influence on the Cn2 estimates.

Free space quantum key distribution using modulating retro-reflectors.

Quantum key distribution (QKD) can be used to produce a cryptographic key whose security is guaranteed by quantum mechanics. The range of fiber-based QKD links is limited, by loss, to a few hundred kilometers, and cannot be used between mobile platforms. Free space QKD can, in principle, overcome these limitations. In practice, very narrow beam divergences must be used, requiring highly accurate pointing of the transmitting terminal to the receiver. This makes deployment very difficult. Here we describe the experimental implementation of a new type of free space QKD link, using modulating retro-reflectors (MRR). The MRR-QKD link eases the pointing requirements by more than three orders of magnitude, from microradians to degrees, while maintaining the narrow beam divergence necessary for long-range communication links. The system uses new, high extinction surface-normal multiple quantum well modulators with a modulation rate of 100 MHz. A laboratory-based BB84 QKD link using multiple quantum well MRRs is demonstrated, link budgets for possible applications are discussed, and security issues are considered.

Suspended photonic waveguide devices.

This article describes recent research at the U.S. Naval Research Laboratory that focuses on the use of micro- and nanomachining techniques for photonic waveguide devices. By selectively etching a sacrificial layer that the waveguide core is supported by, in whole or in part, the waveguide obtains enhanced properties and functionality, such as mechanical flexibility, index contrast, birefringence, and evanescent field depth. We describe how these properties enable unique waveguide applications in areas such as cavity optomechanics, displacement sensing, electro-optics, and nonlinear optics.

InAlAs/InGaAs avalanche photodiode arrays for free space optical communication.

In free space optical communication, photodetectors serve not only as communications receivers but also as position sensitive detectors (PSDs) for pointing, tracking, and stabilization. Typically, two separate detectors are utilized to perform these tasks, but recent advances in the fabrication and development of large-area, low-noise avalanche photodiode (APD) arrays have enabled these devices to be used both as PSDs and as communications receivers. This combined functionality allows for more flexibility and simplicity in optical system design without sacrificing the sensitivity and bandwidth performance of smaller, single-element data receivers. This work presents the development of APD arrays rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of approximately 0.2 at moderate APD gains. We discuss the fabrication and characterization of three types of APD arrays along with their performance as high-speed photodetectors.

Irradiance correlations in retro-reflected beams.

Communications links that utilize modulating retro-reflectors can make use of turbulence-induced fade information available at the remote data-signal terminal in order to optimize the data transfer rate. Experiments were conducted to measure the irradiance in both the direct and the retro-reflected beams. Both on-axis and off-axis components were recorded in order to further study the enhancement in the scintillation index observed in the retro-reflected beam. Measurements were made over a 1.8 km terrestrial range at AP Hill, Virginia. The degree of correlation of the received irradiance between the direct and double-passage beams is found to approach 90% on-axis and 70% outside of the Fresnel zone radius. The scintillation index in the retro-reflected beam is enhanced on-axis due to reciprocal optical paths. The measured scintillation indices, and the correlation of the retro-reflected beam with the direct beam, are compared with a point source, point scatterer, and point receiver model in the strong scintillation approximation.

Mid-infrared difference-frequency generation in suspended GaAs waveguides.

We experimentally demonstrate mid-infrared difference-frequency generation in suspended 181 nm thick GaAs waveguides. Generation of the idler at wavelengths between 2800 and 3150 nm is enabled by form-birefringent phase-matching in ultrahigh index-contrast waveguides. Nonlinear mixing has a measured efficiency of 0.4 W⁻¹ in a 1.2 mm long waveguide using a CW signal tunable between 1490 and 1620 nm and a CW pump tunable between 1018 and 1032 nm at powers of a few mW.

Atmospheric turbulence effects measured along horizontal-path optical retro-reflector links.

The scintillation measured over close-to-ground retro-reflector links can be substantially enhanced due to the correlations experienced by both the direct and reflected echo beams. Experiments were carried out at China Lake, California, over a variety of ranges. The emphasis in this paper is on presenting the data from the 1.1 km retro-reflecting link that was operated for four consecutive days. The dependence of the measured irradiance flux variance on the solar fluence and on the temperature gradient above the ground is presented. The data are consistent with scintillation minima near sunrise and sunset, rising rapidly during the day and saturating at irradiance flux variances of ~10. Measured irradiance probability distributions of the retro-reflected beam are compared with standard probability density functions. The ratio of the irradiance flux variances on the retro-reflected to the direct, single-pass case is investigated with two data sets, one from a monostatic system and the other using an off-axis receiver system.

Probability density of irradiance fluctuations observed over terrestrial ranges.

The irradiance fluctuations imposed on a laser beam that has propagated over horizontal terrestrial paths in the range of 2 to 24 km are compared to lognormal (LN) and gamma-gamma (GG) distributions. For the direct links reported here the irradiance fluctuations follow a LN distribution except in cases of weak turbulence, characterized by a scintillation index of less than 1 and a Fried parameter larger than the receiver aperture, in which case the GG distribution gives an improved fit. In very weak turbulence the difference between the two distributions is insignificant.

Analysis of long-term measurements of laser propagation over the Chesapeake Bay.

Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured for the months of January through June of 2007 on a continuous basis, as conditions allowed. Both the scintillation index sigmaI(2) and the atmospheric structure constant Cn(2) are found to have a strong dependence on the air-minus-water temperature difference. There is no obvious diurnal variation of Cn(2) or of sigmaI(2) akin to the reduction in turbulence level seen in terrestrial links in the hour before sunrise and the hour after sunset. Results from the analysis of these data highlight a need for new approaches to modeling beam propagation in a maritime environment.

Backscatter suppression for underwater modulating retroreflector links using polarization discrimination.

Free space optical links underwater have the potential to enable short range (<100 m) high-bandwidth (megabits per second) data links that have a low probability of detection and interception. The use of a retroreflecting free space optical link in water has the added advantage of allowing much of the weight and power burden of the link to remain at one end. While modulating retroreflectors have been successfully implemented in above-water links, the underwater environment introduces new challenges. The focus of this paper is to address these challenges and to investigate techniques for minimizing their effect on the link performance.