Frank Hanson

High bandwidth underwater optical communication

We report error-free underwater optical transmission measurements at 1 Gbit/s (10(9) bits/s) over a 2 m path in a laboratory water pipe with up to 36 dB of extinction. The source at 532 nm was derived from a 1064 nm continuous-wave laser diode that was intensity modulated, amplified, and frequency doubled in periodically poled lithium niobate. Measurements were made over a range of extinction by the addition of a Mg(OH)(2) and Al(OH)(3) suspension to the water path, and we were not able to observe any evidence of temporal pulse broadening. Results of Monte Carlo simulations over ocean water paths of several tens of meters indicate that optical communication data rates >1 Gbit/s can be supported and are compatible with high-capacity data transfer applications that require no physical contact.

Laser diode side pumping of neodymium laser rods

The laser performance of Nd:YAG, Nd:YLF, and Nd:LLGG has been measured using linear laser diode arrays to side pump 3- mm diam laser rods. We obtained 4.4-mJ output at 1064 nm with Nd:YAG from 16-mJ optical pump pulses 200 micros long at 807- and 3- nm FWHM. The overall electrical to optical efficiency was ~ 6%. Over 0.4 mJ at 532 nm was obtained with a KTP intracavity frequency doubler.

Effects of underwater turbulence on laser beam propagation and coupling into single-mode optical fiber

We characterize and compare the effects of turbulence on underwater laser propagation with theory. Measurements of the coupling efficiency of the focused beam into a single-mode fiber are reported. A simple tip-tilt control system, based on the position of the image centroid in the focal plane, was shown to maintain good coupling efficiency for a beam radius equal to the transverse coherence length, r(0). These results are relevant to high bandwidth communication technology that requires good spatial mode quality.

Off-axis detection and characterization of laser beams in the maritime atmosphere

Remote detection and characterization of laser beams propagating in maritime atmospheres is discussed. A model for off-axis scattered laser light based on Mie scattering from maritime aerosols is presented and compared with angle and time-resolved measurements from a pulsed laser source. We demonstrate that the direction of the source can be determined from the angle-resolved intensity and that the beam direction can be determined from arrival times of the scattered signals if the position of the laser source is known.

Laser propagation at 1.56 μ m and 3.60 μ m in maritime environments

We report results from field experiments that have compared laser propagation at 1.565 μm and 3.603 μm in a variety of atmospheric conditions in a low-altitude maritime environment in order to quantify the relative effects of turbulence under realistic conditions. Intensity scintillation and normalized focused spot sizes were found to be significantly less affected by turbulence at the longer wavelength, in general agreement with theoretical predictions. Also, the longer wavelength beam was noticeably less degraded by aberrations in the transceiver optical components. These advantages should be considered when evaluating the wavelength trade-offs in laser communication systems.

Coherent laser radar at 3.6 µm

Coherent laser radar systems in the mid-IR wavelength region can have advantages in low-altitude environments because they are less sensitive to scattering, turbulence, and humidity, which can affect shorter- or longer-wavelength systems. We describe a coherent laser radar at 3.6 µm based on a single-frequency optical parametric oscillator and demonstrate the system over short ranges outdoors. The system was used to make micro-Doppler measurements from idling trucks that were processed to give surface vibration spectra.

Off-axis laser beam imaging and characterization with two cameras

Off-axis scattering of laser beams propagating in the atmosphere has been imaged by two separated cameras. We give a theoretical analysis and report experiments that show how these images can be used to reconstruct the position and orientation of the beam relative to the cameras. The information from a single image of the beam only determines the beam within a plane. However, the intersection of these planes of ambiguity using images from two cameras can determine the beam uniquely. When the two planes are nearly parallel, an independent method based on the relative radiance at each camera can be used to determine the beam direction.

Comparison of atmospheric laser propagation between the NIR and MWIR

We report results from field experiments that have compared laser propagation in the near infrared (NIR) and mid-wave infrared (MWIR) in a variety of atmospheric conditions. Single frequency laser sources at 1.565 μm and 3.603 μm were transmitted through a single common aperture telescope to ensure that each beam was affected by nearly identical turbulence. Tests were performed on a one-way, 1.26 km path over land and on a round-trip, 2 x 1.41 km path that was mostly over water using a broadband retro-reflector. It is expected from theory that scattering and turbulence should have relatively less effect at longer wavelength, however quantitative measurements in real world conditions are important because of the complexity and simplifying assumptions required in the theory. Although communication and laser radar systems that operate in the NIR at ~1.5 μm benefit from well-developed sources and detectors, it is expected that propagation in the MWIR can offer a significant advantage. The objective of this work was to quantify the relative propagation effects under realistic conditions.