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.

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.

Waveguide micro-opto-electro-mechanical resonant chemical sensors.

We demonstrate a silicon micro-opto-electro-mechanical sensor based on mass-loading of a chemo-selective polymer coated onto a microbridge. The sensor is probed optically using an on-chip waveguide Fabry-Pérot interferometer for high resolution displacement and resonant frequency measurement. The mechanical resonator is designed with paddles to simplify chemo-selective polymer deposition and to minimize any strain effects from the polymer during analyte sorption. Water vapor sensing experiments indicate mass-loading with minimum humidity detection of DeltaRH = 0.25% corresponding to a measured limit-of-detection of 68 parts-per-million. The sensor response time constant is 30 s with minimum frequency noise of 58 Hz at 149.4 kHz resonance. The measured mass-loading resolution is 4.6 picograms. We extract the chemo-selective polymer's partition coefficient and confirm that strong sorption and mass-loading occurs. Various device improvements are discussed, including scaling up to large single-chip sensor arrays, increasing mass-loading resolution by adjusting the device geometry, and using other chemo-selective polymers with larger partition coefficients. These improvements suggest parts-per-billion limit-of-detection levels for a variety of toxic chemicals.

Thermo-optic tuning and switching in SOI waveguide Fabry-Perot microcavities.

Compact silicon-on-insulator (SOI) waveguide thermo-optically tunable Fabry-Perot microcavities with silicon/air Bragg mirrors are demonstrated. Quality factors of Q=4,584 are measured with finesse F=82. Tuning is achieved by flowing current directly through the silicon cavity resulting in efficient thermo-optic tuning over 2 nm for less than 50 mW applied electrical power. The high-Q cavities enable fast switching (1.9 mus rise time) at low drive power (<10 mW). By overdriving the device, rise times of 640 ns are obtained. Various device improvements are discussed.