Marc B. Airola

Experimental Demonstration of a Photonic Analog-to-Digital Converter Architecture With Pseudorandom Sampling

A photonic analog-to-digital converter architecture is demonstrated that uses nonuniform, sub-Nyquist sampling combined with digital signal processing algorithms to unambiguously identify microwave signals of interest. X-band (8-12 GHz) operation is demonstrated with two-tone signals of separation extending to >2 GHz that were directly sampled and digitized at a mean rate of 995.3 MHz resulting in alias-free power spectra.

Long distance laser communications demonstration

AOptix demonstrated a simulated air-to-air laser communications (laser-com) system over a 147Km distance by establishing a laser communication link between the islands of Hawaii and Maui. We expect the atmospheric conditions encountered during this demonstration to be representative of the worst seeing conditions that could be expected for an actual air to air link. AOptix utilized laser-com terminal incorporating Adaptive Optics (AO) to perform high speed tracking and aberration correction to reduce the effects of the seeing. The demonstration showed the feasibility of establishing high data rate point to point laser-com links between aircraft. In conjunction with Johns Hopkins University Applied Physics Laboratory networking equipment we were able to demonstrate a 40Gbit DWDM link, providing significantly more data throughput than is available using RF technologies. In addition to being very high data rate, the link demonstrates very low beam spread, which gives very high covertness, and a high degree of data security. Since the link is based on 1550nm optical wavelengths it is inherently resistant to jamming.

Demonstration of high-data-rate wavelength division multiplexed transmission over a 150-km free space optical link

A 150 km free-space optical (FSO) communication link between Maui (Haleakala) and Hawaii (Mauna Loa) was demonstrated by JHU/APL and AOptix Technologies, Inc. in September 2006. Over a 5 day period, multiple configurations including single channel 2.5 Gbps transmission, single channel 10 Gbps, and four wavelength division multiplexed (WDM) 10 Gbps channels for an aggregate data rate of 40 Gbps were demonstrated. Links at data rates from 10 to 40 Gb/s were run in excess of 3 contiguous hours. Data on the received power, frame synchronization losses, and bit error rate were recorded. This paper will report on the data transfer performance (bit error rates, frame synchronization issues) of this link over a 5 day period. A micropulse lidar was run concurrently, and on a parallel path with the FSO link, recording data on scattering loss and visibility. Comparisons between the state of the link due to weather and the data transfer performance will be described.

Demonstration of dual-polarization fiber ring laser for microwave generation

We demonstrate a photonic technique for the tunable generation of RF, microwave and millimeter wave signals with a single optical source. A dual-polarization supporting and maintaining fiber ring laser is constructed and its use as a tunable RF source is demonstrated.

Free-space optical channel characterization in the maritime environment

The performance of a free-space optical (FSO) communications link in a maritime environment was evaluated during two field trials conducted off the mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Bidirectional, ship-to-shore data links operating at 2.5 Gbps and utilizing commercial, single-mode adaptive optics terminals were set up between a lookout tower located on Cedar Island, VA and a Johns Hopkins University Applied Physics Laboratory research vessel over a range of 2-22 km (optical horizon). This paper presents the test configuration, discusses the overall performance of the FSO channel, and compares it to the available turbulence and weather measurements. Additionally, modeling of the link configuration is presented, and comparisons are made to data measured throughout the experiment such as received power, received beam diameter, and local The performance of a free-space optical (FSO) communications link in a maritime environment was evaluated during two field trials conducted off the mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Bidirectional, ship-to-shore data links operating at 2.5 Gbps and utilizing commercial, single-mode adaptive optics terminals were set up between a lookout tower located on Cedar Island, VA and a Johns Hopkins University Applied Physics Laboratory research vessel over a range of 2-22 km (optical horizon). This paper presents the test configuration, discusses the overall performance of the FSO channel, and compares it to the available turbulence and weather measurements. Additionally, modeling of the link configuration is presented, and comparisons are made to data measured throughout the experiment such as received power, received beam diameter, and local Cn2.

Second-harmonic generation from an array of subwavelength metal apertures

We demonstrate the generation of second-harmonic radiation in transmission through periodic and disordered arrays of sub-wavelength metallic apertures. For circular apertures in a square lattice, the second-harmonic signal peaks at incidence angles corresponding to enhanced transmission of the fundamental beam of 800~nm wavelength except at small incidence angles where the local symmetry minimizes the effective second-order nonlinear susceptibility of the apertures. Even though the linear transmission of the fundamental beam can be more than five times greater through the periodic array as compared to a disordered array, the strength of the second harmonic from the disordered array is greater at large incidence angles. By breaking the local symmetry through the use of apertures of non-centrosymmetric shape, the second-harmonic output occurs at fundamental transmission resonances at small incidence angles.

Water absorption in a refractive index model for bacterial spores

The complexity of biological agents can make it difficult to identify the important factors impacting scattering characteristics among variables such as size, shape, internal structure and biochemical composition, particle aggregation, and sample additives. This difficulty is exacerbated by the environmentally interactive nature of biological organisms. In particular, bacterial spores equilibrate with environmental humidity by absorption/desorption of water which can affect both the complex refractive index and the size/shape distributions of particles - two factors upon which scattering characteristics depend critically. Therefore accurate analysis of experimental data for determination of refractive index must take account of particle water content. First, spectral transmission measurements to determine visible refractive index done on suspensions of bacterial spores must account for water (or other solvent) uptake. Second, realistic calculations of aerosol scattering cross sections should consider effects of atmospheric humidity on particle water content, size and shape. In this work we demonstrate a method for determining refractive index of bacterial spores bacillus atropheus (BG), bacillus thuringiensis (BT) and bacillus anthracis Sterne (BAs) which accounts for these effects. Visible index is found from transmission measurements on aqueous and DMSO suspensions of particles, using an anomalous diffraction approximation. A simplified version of the anomalous diffraction theory is used to eliminate the need for knowledge of particle size. Results using this approach indicate the technique can be useful in determining the visible refractive index of particles when size and shape distributions are not well known but fall within the region of validity of anomalous dispersion theory.

Extinction and backscatter cross sections of biological materials

Aerosol backscatter and extinction cross-sections are required to model and evaluate the performance of both active and passive detection systems. A method has been developed by which begins with laboratory measurements of thin films and suspensions of biological material to obtain the complex index refraction of the film from the UV to the LWIR. Using that result with particle size distribution and shape information as inputs to T-matrix calculations yields the extinction cross-section and backscatter cross section as a function of wavelength. These are important inputs to the lidar equation. In a continuing effort to provide validated optical cross-sections, measurements have been made on a number of high purity biological species in the laboratory as well as measurements of material released at recent field tests. The resulting observed differences aid in distinguishing between intrinsic and extrinsic effects, which can affect the characteristic signatures of important biological aerosols. A variety of biological aerosols are examined.

Spectral Refractive Index and Extinction Cross-Section of BG Spores

Despite the wide spread need for optical cross-section data on single spore bio-aerosols, available databases are sparse and unreliable. Information reported is based on short path measurements on high concentration media containing particle clusters. This represents an upper bound to the single spore cross-section. Measurements on single spore aerosolized media demand long path lengths and moderate particle concentration. Transmittance measurements need to be in the single scatter limit as well. These requirements are often difficult to meet. We present a procedure that leads to aerosol extinction and backscatter cross-sections in a straightforward manner. Transmittance measurements of thin films of bio-aerosols are used to obtain the bulk refractive index. This result and the measured size distribution can be used in a T-matrix calculation to yield the desired cross-sections. To illustrate this technique, infrared cross-sections are obtained for Bacillus globigii.

Temperature dependent BRDF facility

Applications involving space based instrumentation and aerodynamically heated surfaces often require knowledge of the bi-directional reflectance distribution function (BRDF) of an exposed surface at high temperature. Addressing this need, the Johns Hopkins University Applied Physics Laboratory (JHU/APL) developed a BRDF facility that features a multiple–port vacuum chamber, multiple laser sources covering the spectral range from the longwave infrared to the ultraviolet, imaging pyrometry and laser heated samples. Laser heating eliminates stray light that would otherwise be seen from a furnace and requires minimal sample support structure, allowing low thermal conduction loss to be obtained, which is especially important at high temperatures. The goal is to measure the BRDF of ceramic-coated surfaces at temperatures in excess of 1000°C in a low background environment. Most ceramic samples are near blackbody in the longwave infrared, thus pyrometry using a LWIR camera can be very effective and accurate.