Lasers offer tremendous advantages over RF communication systems in bandwidth and security. Atmospheric
turbulence causes severe received power variations and high bit error rates (BERs) in airborne laser communication.
If two or more laser beams are separated sufficiently, they can effectively "average out" the effects
of the turbulence. This requisite separation distance is derived for multiple geometries, turbulence conditions,
and turbulence effects. Integrating multiple techniques into a system alleviates the deleterious effects of turbulence
without bulky adaptive optics systems. Wave optics simulations show multiple transmitters, receiver and
transmitter trackers, and adaptive thresholding significantly reduce BER (by over 10,000 times).
Lasers offer tremendous advantages over RF communication systems in bandwidth and security, due to their ultrahigh
frequency and narrow spatial beamwidth. Unfortunately, atmospheric turbulence causes severe received
power variations and significant bit error rates (BERs) in free-space optical communication (FSOC). Airborne
optical communication systems require special considerations in size, complexity, power, and weight. We alleviate
the deleterious effects of turbulence by integrating multiple techniques into an on/off keying direct detection
system. Wave optics simulations show a combination of transmitter diversity, receiver and transmitter trackers,
and adaptive thresholding significantly reduces the BER in air-to-air FSOC (up to 13 dB). Two transmitters
alone provide a significant BER improvement over one transmitter, especially for the strong turbulence regime
with up to a 9 dB improvement. Two beams also provide a reduction in fade length, indicating they will probably
provide even greater improvement with interleaving and forward error correction coding.
Analog-to-digital converters (ADCs) are an essential component of digital receiver systems. Progress at advancing the electronic ADC modules has been very slow due in large part to the difficulties in fabricating the electronic circuitry required for very high resolution and high sampling rate converters. This slow progress has resulted in a bottleneck between the received analog signal and the digital signal processing system. Single or multiple analog signal down conversion stages are required in digital receivers to down convert the received analog signal to an intermediate frequency (IF) that can be processed by the electronic ADC. There has been much recent interest in the use of photonics for direct digitization of the analog signal at the received RF frequency thus eliminating the need for analog down conversion. This paper reviews some of the recent research advancements in photonic ADCs. We will especially focus on the development of a novel photonic ADC module that uses semiconductor saturable absorbers to perform the data quantization. We will also present recent results in the development of a mode-locked fiber laser used as the sampling source in this photonic ADC architecture.
Precise control of the dispersion within mode-locked laser cavities can lead to optical pulse compression and reduced timing jitter of mode-locked lasers. Two simple measurement techniques are used to provide a complete picture of the dispersion within an erbium doped mode-locked fiber laser cavity. We measured the optical dispersion of erbium-doped fiber, standard single mode fiber, and chirped Bragg gratings. We built a Michelson interferometer with a wideband LED source to measure the dispersion of fiber lengths of less than 1 meter. Next, we measured the dispersion of chirped Bragg gratings using a network analyzer and a tunable laser in a differential phase measurement technique.
A new atmospheric screen generator is developed for use in performance calculations of adaptive optics and imaging systems. The generator is valid over a wide range of atmospheric turbulence parameters and incorporates both phase and amplitude effects. The new screen generator accounts for diffraction effects caused by turbulence and incorporates the phase, amplitude, and cross statistics of a weak turbulence model. The second order statistics of the phase and amplitude perturbations are based on the auto- correlation functions developed by Lee and Harp and the cross-correlation of the phase and amplitude perturbations derived in this paper. The correlations are derived by modeling the turbulence as a number of layers of randomly varying refractivity perpendicular to the propagation path. As the field propagates through the medium, diffraction occurs at each of the layers. A Fourier series expansion of the wavefront phase and amplitude is used. The screen generator uses the power and cross spectral densities of the phase and amplitude perturbations. The mean square value and the structure functions of the wavefront phase and amplitude are calculated in a Monte Carlo experiment and shown to be within 1% of the theoretical value.