Kerry A. Mudge

Scintillation index of the free space optical channel: Phase screen modelling and experimental results

Scintillation index (SI) is a key metric for free space optical communications (FSOC), and measures the normalised intensity variance caused by atmospheric turbulence. It is a function of the refractive index structure parameter Cn2, range, and receiver aperture. There is a need for an atmospheric simulation model of the effects of scintillation because testing of FSOC performance in the environment is difficult and time consuming. In this paper we compare experimental results with numerical simulations using phase screens for channels involving three receivers of different size apertures. There is good agreement in the results of experiment and model.

Comparison of probability density functions for analyzing irradiance statistics due to atmospheric turbulence

A large number of model probability density functions (PDFs) are used to analyze atmospheric scintillation statistics. We have analyzed scintillation data from two different experimental setups covering a range of scintillation strengths to determine which candidate model PDFs best describe the experimental data. The PDFs were fitted to the experimental data using the method of least squares. The root-mean-squared fitting error was used to monitor the goodness of fit. The results of the fitting were found to depend strongly on the scintillation strength. We find that the log normally modulated Rician and the log normal PDFs are the best fit to the experimental data over the range of scintillation strengths encountered.

Development and implementation of a robust angle of arrival turbulence measurement system

Abstract. Free space optical communications (FSOC) systems are a promising complement to existing wireless communications technologies. FSOC systems have many significant advantages over traditional radio frequency links, including high bandwidth, no spectrum licensing requirements, low-power consumption, small payloads, low probability of intercept, and greater immunity from interference or jamming. However, atmospheric turbulence (scintillation) imparts significant phase noise onto the laser beam, resulting in intensity fluctuations at the receiver. In order to develop scintillation mitigation strategies, it is necessary to monitor scintillation in parallel to the communications channel. We report on the development and implementation of a robust angle of arrival (AoA) turbulence measurement instrument that is suitable for this task. Several key data acquisition and processing techniques were designed to enhance the reliability and robustness of the scintillation measurement.

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.

Real-time scintillation noise mitigation for free space optical transmission of analogue and digital signals

It is well-known that free space optical communications through a turbulent atmosphere are adversely affected by scintillation noise. This paper reports on the experimental demonstration of a two-colour common mode rejection technique to mitigate atmospheric scintillation noise. Real-time equalisation was achieved for both analogue (amplitude modulated PAL composite video) and digital (quadrature amplitude modulated) signals.

Development and characterisation of FPGA modems using forward error correction for FSOC

In this paper we report on the performance of a free-space optical communications (FSOC) modem implemented in FPGA, with data rate variable up to 60 Mbps. To combat the effects of atmospheric scintillation, a 7/8 rate low density parity check (LDPC) forward error correction is implemented along with custom bit and frame synchronisation and a variable length interleaver. We report on the systematic performance evaluation of an optical communications link employing the FPGA modems using a laboratory test-bed to simulate the effects of atmospheric turbulence. Log-normal fading is imposed onto the transmitted free-space beam using a custom LabVIEW program and an acoustic-optic modulator. The scintillation index, transmitted optical power and the scintillation bandwidth can all be independently varied allowing testing over a wide range of optical channel conditions. In particular, bit-error-ratio (BER) performance for different interleaver lengths is investigated as a function of the scintillation bandwidth. The laboratory results are compared to field measurements over 1.5km.

Design of a coupled quantum well modulator with enhanced modulation efficiency

A coupled quantum well modulator design is presented which offers a 30% improvement in modulation efficiency at 1.5μm, compared to current state-of-the-art [1].

Laser communication of FM audio/video signals using InGaAs modulating retro-reflectors

We report on the free space optical transmission of FM audio/video signals using a 6.3mm diameter InGaAs modulating retro-reflector.

Long Range Audio Laser Communication Link in a Maritime Environment

The Defence Science & Technology Organisation (DSTO), in collaboration with the US Naval Research Laboratory (NRL), has performed long distance experiments on analogue modulated free space optical links across Chesapeake Bay, Maryland. In the present work, pulse frequency modulation was used to transmit audio signals over a distance of 32 km (folded path across the Bay). Still images were transmitted using slow scan television (SSTV) techniques, and a novel technique to decrease the transmission time of SSTV images is presented.

Audio communications with a mid-IR laser

We have demonstrated audio communications with a mid-IR laser. The laser is a frequency doubled Q-switched CO2 system producing approximately 12ns pulses at 4.6μm. The audio signal was encoded on the beam by means of pulse frequency modulation (PFM) with a carrier frequency of 37kHz. A 1mm diameter, low noise thermoelectrically cooled IR photovoltaic detector with electrical bandwidth 250MHz was used to detect the laser beam. A custom-built circuit stretched the resultant electrical pulses to approximately 1.5μs, before being demodulated. High quality audio signals were received and recorded, and still images were successfully transmitted using slow scan television techniques. The demonstration was conducted at the Defence Science & Technology Organisations laser range at Edinburgh, South Australia in July 2008. The distance was 1.5km, with a slant path (8m to 1.5m). The maximum range using this system is estimated to be tens of kilometres.