Carlos O. Font

Free space optical communications research at the U.S. Naval Research Laboratory

Free space optical (FSO) communication has enjoyed a renewal of interest in the past decade driven by increasing data rate requirements and decreasing amounts of radio frequency spectrum. These needs exist in both the commercial and military sectors. However military communications requirements differ in other ways. At the U.S. Naval Research Laboratory (NRL) we have been conducting research on FSO communications for over ten years with an emphasis on tactical applications. NRLs FSO research has covered propagation studies in the maritime domain, new component development, and systems demonstrations. In addition NRL has developed both conventional, direct, laser communications systems and retro-reflecting systems. In this paper we review some of this work and discuss possible future applications of FSO communications.

Humidity's influence on visible region refractive index structure parameter C n 2

In the near-infrared and visible bandpasses optical propagation theory conventionally assumes that humidity does not contribute to the effects of atmospheric turbulence on optical beams. While this assumption may be reasonable for dry locations, we demonstrate that there is an unequivocal effect owing to the presence of humidity upon the strength of turbulence parameter, C(n)(2), from data collected in the Chesapeake Bay area over 100 m length horizontal propagation paths. We describe and apply a novel technique, Hilbert phase analysis, to the relative humidity, temperature, and C(n)(2) data to show the contribution of the relevant climate variable to C(n)(2) as a function of time.

Comparing horizontal path C2n measurements over 0.6 km in the tropical littoral environment and in the desert

We have measured the optical turbulence structure parameter, C2n, in two extremely different locations: the first being the littoral region on the southwest coast of Puerto Rico. The second location is over the dry desert in central New Mexico. In both cases, the horizontal beam paths are approximately 0.6 km long, within 2 meters of the local surface (Puerto Rico) and varying between 2 to 100 meters (New Mexico). We present our findings from the two datasets.

Implementation of a phase-only spatial-light modulator for an atmospheric turbulence simulator at the short wavelength infrared regime

Modeling and simulating the atmosphere in a controlled environment has been a study of interest to scientists for decades. The development of new technologies allows scientists to perform this task in a more realistic and controlled environment and provides a powerful tool for the study and better understanding of the propagation of light through the atmosphere. Technologies like Free-space laser communications (FSLC) and/or studies on light propagation through the atmosphere are areas which constantly benefit from breakthroughs in the development of atmospheric turbulence simulators. In this paper we present the results of the implementation of a phase only spatial light modulator (SLM) as an atmospheric turbulence simulator at the Short-Wave Infra-Red (SWIR) regime and its use with a FSLC system.

Long range, analog RF free space optical communication link in a maritime environment

The Naval Research Laboratory (NRL) in collaboration with the Defence Science and Technology Organisation (DSTO) of Australia has performed long distance experiments with analog modulated free space optical communication links across the Chesapeake Bay. Results will be presented on estimating the probability density functions of the RF parameters of gain, noise factor, and linearity after propagating an RF modulated, 1550nm laser beam over a 32km distance (folded round-trip across Chesapeake Bay). In addition, results from the transmission of video using analog FM modulation of a 1550nm laser beam over the link will be presented.

Study of optical phase change measurement using the Hilbert Transform and interferometric techniques

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.

FM-MRR Analog Audio Transmission through a Scintillated Channel

In this work, a free-space infrared communications system is described. The system has the capability of using previously captured scintillation data and introducing the effects onto the bench-top system. This effectively acts as a scintillation simulator which emulates an optical link that is effected by the weather and various physical conditions at the time of transmission. The method used for scintillation simulation is described. The transmission method of the system is a hybrid combination of traditional frequency modulation (FM) and optical amplitude modulation (OAM) combined with Multiple Quantum Well (MQW) Modulating Retroreflector (MRR) technology. The result has produced a robust, low power system that is capable of transmitting real-time audio information with high clarity along a channel that accurately simulates the atmospheric effects of scintillation. The system is capable of transmitting along a link of several kilometers, depending specifically on the characteristics of the interrogator and sensor components chosen for the system.

Characterization and training of a piezoelectric deformable mirror for operation at 1550 nm

Characterizing the fundamental response and operational parameters of a deformable mirror is a critical first step in the design of an adaptive optics system. This paper describes the characterization of the influence function and training of a piezoelectric deformable mirror (PDM) at 632 nm. We scale the results to 1550 nm for low to mid order aberration correction for free space laser communications applications in the Short Wave Infrared (SWIR). A modified Twyman-Green interferometer was used to measure the influence functions and to characterize the mirror. The data was analyzed using commercial and customized software.

Humidity contribution to C2n over a 600m pathlength in a tropical marine environment

We present new optical turbulence structure parameter measurements, C2n, over sea water between La Parguera and Magueyes Island (17.6N 67W) on the southwest coast of Puerto Rico. The 600 meter horizontal paths were located approximately 1.5 m and 10 m above sea level. No data of this type has ever been made available in the literature. Based on the data, we show that the C2n measurements are about 7 times less compared to equivalent land data. This strong evidence reinforces our previous argument1-4 that humidity must be accounted for to better ascertain the near surface atmospheric turbulence effects, which current visible / near infrared C2n bulk models fail to do. We also explore the generalised fractal dimension of this littoral data and compare it to our reference land data. We find cases that exhibit monofractal characteristics, that is to say, the effect of rising temperatures during the daylight hours upon turbulence are counterbalanced by humidity, leading to a single characteristic scale for the measurements. In other words, significant moisture changes in the measurement volume cancels optical turbulence increases due to temperature rises.

Adaptive transmission algorithms for a hard-switched FSO/RF link

Hybrid communication systems exploit channel diversity to improve network performance. One such example is hybrid Free Space Optical (FSO)/Radio Frequency (RF) systems. These networks aim to merge the high capacity of FSO links with the high availability of RF links to form resilient high-bandwidth communications. When used for wireless communications, the main challenge is the dynamic response to variations in the atmospheric channel in adverse weather conditions. In this paper we simulate adaptive hard-switched transmission algorithms on a practical prototype hybrid system. FSO channel scenarios are formed based on climate data, and are emulated on the prototype system to compare the performance of the hybrid system to each solo carrier respectively.