Kerry A. Corbett

Characterizing the propagation path in moderate to strong optical turbulence

In February 2005 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation and the University of Central Florida. A Gaussian beam was propagated along a horizontal 1500 m path near the ground. Scintillation was measured simultaneously at three receivers of diameters 1, 5, and 13 mm. Scintillation theory combined with a numerical scheme was used to infer the structure constant C2n, the inner scale l0, and the outer scale L0 from the optical measurements. At the same time, C2n measurements were taken by a commercial scintillometer, set up parallel to the optical path. The C2n values from the inferred scheme and the commercial scintillometer predict the same behavior, but the inferred scheme consistently gives slightly smaller C2n values.

Scintillation : theory vs. experiment

In May 2004 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation, the Office of Naval Research and the University of Central Florida. A 45 mm divergent Gaussian beam was propagated along a horizontal 1500 meter path approximately 2 meters above the ground. At the receiver were 3 apertures of diameter 1mm, 5mm, and 13mm. The scintillation was measured at each aperture and compared to scintillation theory, recently developed for all regimes of optical turbulence. Three atmospheric parameters, Cn2, lo and Lo, were inferred from these optical measurements. Simultaneously, a commercial scintillometer, which recorded values for Cn2, was set up parallel to the optical path. In this paper, a numerical scheme is used to infer the three atmospheric parameters and comparisons are made with the Cn2 readings from the scintillometer.

Differential oversampling data converters in SEED technology

Abstract Differential architectures for both first order error diffusion and first order sigma–delta modulators are presented in this paper. Techniques required to transform single-ended architectures to differential architectures are discussed which are suitable for implementation in both p–i–n and n–i–n SEED technologies. Descriptions of common SEED circuit modules, together with SPICE behavioural simulations are also presented. A feature of the architectures presented is that they can be fully integrated into a single substrate using MEMS technology. This can be done by incorporating integrated optical waveguides together with MMIC technology. The goal of this work is a fully integrated differential optical oversampling modulator with extremely high resolution and linearity.

Initial Measurements of Atmospheric Parameters in a Marine Environment

April 2005, a laser propagation experiment was conducted over a 470m horizontal maritime path. Scintillation measurements of a divergent Gaussian beam wave were taken simultaneously for different receiver aperture sizes. Terrestrial scintillation theory combined with a numerical algorithm was used to infer the atmospheric parameters Cn2 and lo from the optical maritime scintillation measurements. This paper presents the initial results.

Numerical experiments in atmospheric scintillation correlation for applications in dual channel optical communications

A method for reducing noise in near-IR laser communications has been proposed that relies upon the dual wavelength output of the He-Xe laser having a high level of noise coherence. However, in transmissions through the atmospheric boundary layer, an additional and significant noise component is added by atmospheric scintillation. These scintillations are mainly limited to frequencies of less than 1 kHz and are correlated in the two laser channels to a degree determined by the channel wavelength separation, the transmission range and the severity of the turbulence regime. To analyze the propagation of waves in random media one normally considers the statistics of the field. In the case of small angle forward scattering, which is the case of interest in laser propagation, field moments higher than the fourth are so difficult to solve that no solutions are known outside of the asymptotic weak and strong approximations. An alternative approach is to conduct numerical experiments in which one generates a realization of the random medium (with the desired statistics) and then calculates the wave field. We have numerically modeled the spatial irradiance intensity as a function of range from a point source under turbulence regimes typical of daytime conditions near the Earth’s surface. Simulations were performed for two closely separated channels in the near-IR (1556.5 and 1558.1 nm). We present the results of these simulations together with the implications for the mitigation of atmospheric scintillation noise by common mode rejection.

Mitigation of scintillation noise in a 32-km maritime path

We present a technique to correct for atmospheric scintillation noise in free space optical communications. Co-channel noise is removed by common mode rejection, where each channel is transmitted on separate, but closely spaced, wavelengths.

PDF models of the irradiance fluctuations in Gaussian beam waves

The lognormal and gamma-gamma distributions are compared to simulated and experimental data of the irradiance fluctuations of a Gaussian beam wave propagating through the atmosphere on a horizontal path, near ground, in the moderate-to-strong turbulence regime. Irradiance data was collected simultaneously at three receiving apertures of different size. Atmospheric parameters were inferred from the measurements and used to reproduce the experimental data with numerical simulations and calculate the parameters for the theoretical probability density functions (PDFs). The simulation values agree well with the experimental data for all three aperture sizes, while the support for the theoretical PDFs depends on the size of the receiving aperture.

Optical threshold logic analog-to-digital converters using self electro-optic effect devices

A novel 2-bit optical-input optical-output analog-to-digital converter (ADC) is demonstrated in self electro-optic device (SEED) technology using a threshold logic technique. The threshold gate was constructed using a resistor-SEED (R-SEED) which is composed of a large value resistor and a SEED area of 500 um x 500 um. Each gate operates as a majority function that has a threshold level controlled by a fixed optical input. The ADC was constructed using two R-SEED gates operating at wavelength of 846 nm. The test bench set-up operates at 100 Hz. However, as the proposed architecture is scalable, it can operate at much higher speeds and generate larger number of bits. This architecture is only limited by the switching speed of the SEED and propagation delay through each threshold gate.

Performance of a photonic oversampled sigma-delta quantiser

In an increasingly digital world, the need for high speed and high fidelity analog-to-digital (A/D) converters is paramount. Performance improvements in electronic A/Ds have not kept pace with demand, hence the need to consider alternative technologies. One such technology is photonics, as it takes advantage of optical sampling, high speed optical switches and low cross-talk interconnects. Optical sampling derives its advantage from the application of ultra low timing jitter (<100fs) mode locked lasers utilised to provide high speed clock pulses. In this paper we report on our investigation into the feasibility and simulated performance of a photonic sigma-delta quantiser. The first-order sigma-delta architecture requires the functional elements of a subtractor, comparator and delay. We constructed optoelectronic versions of a subtractor and a comparator using self-electro-optic devices (SEED) based upon multiple quantum well (MQW) p-i-n devices. Comparator and subtractor operation were experimentally demonstrated and the inclusion of gain was shown to improve the subtractor performance to that demanded by the sigma-delta architecture. A numerical simulation based upon experimentally derived data was performed to include the non-idealities of the comparator and subtractor. A photonic implementation of the sigma-delta was proposed and simulated to demonstrate the feasibility of the architecture design and to determine the signal-to-quantisation-noise ratio (SQNR) as a function input amplitude. A peak SQNR of 54dB was obtained for an oversampling ratio of 100.

Initial results on the fabrication of long-period fiber Bragg gratings with a CO2 laser.

Long-period fiber Bragg gratings (LPG) where the grating period is much longer than the wavelength of light have many unique characteristics and find uses in gain-flattening filters and mode converters. This paper describes the characteristics of the initial LPGs fabricated at the University of Adelaide using an infrared CO2 laser. The optical system implemented promotes uniform irradiation of the full circumference of the fiber, avoiding many of the non-uniformities, associated with a single sided system. Some initial gratings have been made using this method, which typically show an attenuation of 10dB within a wavelength range (FWHM) of 8 nm. Work is now focused on improving these devices through an understanding of the writing process and its effect on the transmitted spectrum.