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.
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.
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.
KEYWORDS: Frequency modulation, Fermium, Video, Modulation, Free space optics, Signal to noise ratio, Receivers, Analog electronics, Sensors, Modulators
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.
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 Organisation's 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.
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.
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.
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.
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.
Mean irradiance data from a field experiment conducted jointly by the Australian Defence Science and Technology Organisation, the Office of Naval Research, and the University of Central Florida is presented. The experiment was conducted in May 2004 in Adelaide, Australia. The propagation path was characterized by conditions of moderate to strong irradiance fluctuations. The data is compared to existing theoretical results and a new theoretical result developed in this paper. The new theoretical result is based on a modified Rytov method that extends the validity of the Rytov method into moderate to strong irradiance fluctuation conditions.
There is increasing interest in free space optical communications as an alternative to fibre optics and radio frequency communications, particularly in 'last mile' applications and applications with weight and power restrictions e.g. communications with unmanned aerial vehicles. The potential advantages of free space optical communications include: high bandwidth; no licensing issues; smaller, lighter payloads; low probability of intercept; and immunity from interference/jamming. However, propagation through the atmosphere is subject to atmospheric scintillation noise affecting the signal-to-noise ratio (SNR), effectively reducing the range and bandwidth of the communication link. This scintillation is experienced even over relatively short propagation paths and is caused by small temperature variations in the atmosphere, resulting in index of refraction changes. In this paper we present a technique to correct for atmospheric scintillation noise in free space optical communications and laser remote sensing. It uses common mode rejection to remove co-channel noise, where each channel is transmitted on separate, but closely spaced, wavelengths. The signal-to-noise ratio is significantly increased, thereby increasing the range and/or bandwidth of the link. To date, tests have been conducted with analogue audio and video transmissions. This has been successful, with improvements of up to 12dB in SNR having been demonstrated. This has been limited by the current implementation, which is only at prototype stage -- the ultimate achievable improvement in SNR is anticipated to be significantly higher.
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.
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.
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.
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.
As the demand for analog-to-digital (A/D) conversion with greater bandwidths increase, it is necessary to look at other alternatives to electronics for integrated circuit design. One such approach to utilize is a combination of optics and electronics, or opto-electronics, at all levels of the system hierarchy. A device that has these properties is the Self Electro-optic Effect Device (SEED), and combining this with oversampling techniques for data conversion can meet the demands for direct digitization of radio frequency (RF) signals. One form of A/D oversampling conversion method is Sigma-Delta modulation. A key element of this technique is the subtractor and in this paper we will discuss the implementation of a differential subtractor using SEEDs as part of a Sigma-Delta Modulator. This paper will detail simulation results based on experimental data to predict the behavior of two types of differential subtractors, one of which will be compared with experimental results.
In this article photonic implementations of two oversampled analog-to-digital converter architectures are discussed. The first, and simplest design is that of pulse-code-modulation. Simulations and an experiment are developed employing a Multiple-Quantum-Well p-i-n diode comparator. Agreement between simulation and experiment is demonstrated and the design is subsequently extended to a higher performance first order unipolar sigma-delta architecture. Results of the signal-to-noise ratio as a function of input amplitude are presented for an oversampling ratio of 100.
KEYWORDS: Modulators, Modulation, Analog electronics, Quantum wells, Signal to noise ratio, Absorption, Optoelectronics, Electro optics, Data conversion, Diodes
Oversampled analog-to-digital conversion architectures provide a trade-off between sampling speed for improved amplitude resolution without the need for complex analog circuits. One form of oversampled data conversion techniques is sigma-delta (ΣΔ) modulation, which takes advantage of high sampling rates. In this paper, a performance evaluation on ΣΔ modulators based on measured data of Self Electro-optic Effect Devices are presented and discussed. The data are used in evaluating the performance of the A/D converter using MATLAB simulations.
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