Tech Briefs

A laser beam transmitter array could propagate more effectively through weak or strong atmospheric turbulence.

This work explored the concept of creating a partially coherent laser beam consisting of an array of spatially overlapping or separated Gaussian beams with possible individual control of each individual emitter’s wavelength. The idea was to test whether such a transmitter array could propagate more effectively through weak or strong atmospheric turbulence. It was proposed that a versatile, multi-wavelength, multi-emitter configuration could be realized via an array of optically pumped, vertical external-cavity surface emitting semiconductor lasers (VECSELs).

The Multi-Wavelength, Multi-Emitter Configuration could be realized via an array of optically pumped VECSELs. The schematic shows an overlapping array of individual spatially overlapping transmitter beams (left) or spatially separated transmitter beams (right) individually capable of operating at different wavelengths.
Results to date show record power outputs near 40 W in a TEM00 mode, and near 64 W in multi-lateral mode. As a result of the theoretical analysis carried out and a validation experiment using much lower power fiber lasers in a laboratory setting, such a transmitter array is feasible.

The VECSEL geometry is compact, with a surface outcoupled TEM00 beam, and is readily tunable over a 30-50 nm bandwidth. In the compact VECSEL cavity, the diode pumps are integrated onto a heat sink with the VECSEL active mirror. At high pump power densities, the quantum wells in the resonant periodic gain stack within the VECSEL structure are inverted and emit light at the signal wavelength. The external output coupler provides optical feedback and transmits the outcoupled TEM00 beam. The birefringent filter in the cavity acts as a tuning element to control the output signal wavelength.

As a first step, an array of Gaussian beams was considered theoretically and an analysis was conducted for propagation in both weak and strong turbulence. In the transmitter-receiver geometry, a slow detector was assumed and the longitudinal and radial components of the scintillation index were calculated for a typical free-space communications laser setup.

Calculations assuming weak optical turbulence show a significant reduction in the scintillation index for multiple beams. This reduction is calculated as a function of different beam center-to-center spacing. A reduction of longitudinal scintillation index of greater than 92% is predicted for nine beams at the transmitter when the separation and beam spot size has been optimized.

In the laboratory experiment, the multi-emitter beam was generated by spatially combining several beams from single-mode fibers. The results confirmed the theory prediction of scintillation index for weak turbulence and, moreover, showed that a similar reduction is observed under strong turbulence conditions. The beam diameter of the individual emitters was around 0.42 mm, and a phase screen was placed in the beam path to simulate weak and strong turbulence. The total path length in the laboratory was 2 meters.

The experimental data verify the theoretical predictions and show that the partial spatial coherence was dominant over the multi-color diversity in the beam(s). This sequence of results suggests that field tests with the fiber laser array replaced by a VECSEL array, with each VECSEL outputting multi-Watts of power at selected wavelengths, is a viable modality for laser communications in a turbulent atmosphere.

The large separation of scales between the wavelength of the incident light and individual nano features led to the implementation of an adaptive space and time FDTD algorithm. Many attempts have been made to develop stable, second-order, accurate adaptive space-time FDTD schemes. The challenge in maintaining algorithm stability and second-order accuracy was in implementing appropriate extrapolation schemes between fine and coarse time and space mesh boundaries.

Another issue addressed was that of avoiding staircasing effects on surfaces due to the fact that the global coarse grid on the large computation domain does not generally conform to the shape of an arbitrary surface. In realistic simulations, one does not have the luxury of assuming an ideal numerical grid that satisfies a specific symmetry as objects with arbitrary complex shapes within the computational domain are discovered. A number of schemes involving nonorthogonal grids were explored to achieve smooth transitioning between the global Cartesian and a numerical grid that conforms to the surface of interest. The latter grid would avoid numerical artifacts associated with staircasing on surfaces — for example, the surface plasmonic modes generated on a metallic nanostructure. As stressed above, these near-fields profoundly influence objects (atoms, quantum dots, etc.) that may be attached to the surface of only a few nanometers away.

This work was done by the Arizona Board of Regents, University of Arizona, for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. AFRL-0151

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Computational Photonics in Laser Communications Through Clouds (reference AFRL-0151) is currently available for download from the TSP library.

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