Showing papers on "Optical communication published in 2005"

Chaos-based communications at high bit rates using commercial fibre-optic links

Abstract: Chaos is good, if you are looking to send encrypted information across a broadband optical network. The idea that the transmission of light-based signals embedded in chaos can provide privacy in data transmission has been demonstrated over short distances in the laboratory. Now it has been shown to work for real, across a commercial fibre-optic channel in the metropolitan area network of Athens, Greece. The results show that the technology is robust to perturbations and channel disturbances unavoidable under real-world conditions. Chaotic signals have been proposed as broadband information carriers with the potential of providing a high level of robustness and privacy in data transmission1,2. Laboratory demonstrations of chaos-based optical communications have already shown the potential of this technology3,4,5, but a field experiment using commercial optical networks has not been undertaken so far. Here we demonstrate high-speed long-distance communication based on chaos synchronization over a commercial fibre-optic channel. An optical carrier wave generated by a chaotic laser is used to encode a message for transmission over 120 km of optical fibre in the metropolitan area network of Athens, Greece. The message is decoded using an appropriate second laser which, by synchronizing with the chaotic carrier, allows for the separation of the carrier and the message. Transmission rates in the gigabit per second range are achieved, with corresponding bit-error rates below 10-7. The system uses matched pairs of semiconductor lasers as chaotic emitters and receivers, and off-the-shelf fibre-optic telecommunication components. Our results show that information can be transmitted at high bit rates using deterministic chaos in a manner that is robust to perturbations and channel disturbances unavoidable under real-world conditions.

Free-space optical MIMO transmission with Q-ary PPM

Abstract: The use of multiple laser transmitters combined with multiple photodetectors (PDs) is studied for terrestrial, line-of-sight optical communication. The resulting multiple-input/multiple-output channel has the potential for combatting fading effects on turbulent optical channels. In this paper, the modulation format is repetition Q-ary PPM across lasers, with intensity modulation. Ideal PDs are assumed, with and without background radiation. Both Rayleigh and log-normal fading models are treated. The focus is upon both symbol-/bit-error probability for uncoded transmission, and on constrained channel capacity.

Security performance of optical CDMA Against eavesdropping

Abstract: Enhanced security has often been cited as an important benefit of optical CDMA (O-CDMA) signaling. However, the quality and degree of security enhancement has not been closely examined in the literature. This paper examines the degree and types of security that may be provided by O-CDMA encoding. A quantitative analysis of data confidentiality is presented for O-CDMA encoding techniques that use both time spreading and wavelength hopping. The probability of successful data interception is calculated as a function of several parameters, including signal-to-noise ratio and fraction of total available system capacity. For reasonable choices of system and encoding parameters, it is shown that increasing code complexity can increase the signal-to-noise ratio (SNR) required for an eavesdropper to "break" the encoding by only a few dB, while the processing of fewer than 100 bits by an eavesdropper can reduce the SNR required to break the encoding by up to 12 dB. The overall degree of confidentiality obtainable through O-CDMA encoding is also compared with that obtainable through standard cryptography. time-spreading/wavelength-hopping in particular, and O-CDMA in general, are found to provide considerably less data confidentiality than cryptography, and the confidentiality provided is found to be highly dependent on system design and implementation parameters.

All-optical switching in rubidium vapor.

Abstract: We report on an all-optical switch that operates at low light levels. It consists of laser beams counterpropagating through a warm rubidium vapor that induce an off-axis optical pattern. A switching laser beam causes this pattern to rotate even when the power in the switching beam is much lower than the power in the pattern. The observed switching energy density is very low, suggesting that the switch might operate at the single-photon level with system optimization. This approach opens the possibility of realizing a single-photon switch for quantum information networks and for improving transparent optical telecommunication networks.

Performance of APD-based, PPM free-space optical communication systems in atmospheric turbulence

Abstract: In this paper, we characterize the performance of a direct-detection, avalanche photodiode-based free-space optical (FSO) communication system in terms of the overall bit-error rate. The system of interest uses pulse-position modulation (PPM) and is subjected to scintillation due to optical turbulence. Two scenarios are considered. In one case, a weak turbulence (clear-air) scenario is considered, for which the received signal intensity may be modeled as a log-normal random process. In the other case, we consider a negative exponentially distributed received signal intensity. To arrive at the desired results, it is assumed that the system uses a binary PPM (BPPM) modulation scheme. Furthermore, it is assumed that the receiver thermal noise is nonnegligible, and that the average signal intensity is large enough to justify a Gaussian approximation at the receiver. Union bound is used to assess the performance of M-ary PPM systems using the results of the BPPM scenario. Numerical results are presented for the BPPM case to shed light on the impact of turbulence on the overall performance.

Alamouti-type space-time coding for free-space optical communication with direct detection

Abstract: A modification of the Alamouti code originally proposed for RF wireless applications is described that allows it to be applied in scenarios such as free-space optical communication with direct detection where unipolar modulations like pulse-position modulation and on-off keying are traditionally used to convey the information. The modification of the code and associated decision metric is such as to maintain all of the desirable properties of the original scheme.

Optical repetition MIMO transmission with multipulse PPM

Abstract: We study the use of multiple laser transmitters combined with multiple photodetectors for atmospheric, line-of-sight optical communication, and focus upon the use of multiple-pulse-position-modulation as a power-efficient transmission format, with signal repetition across the laser array. Ideal (photon counting) photodetectors are assumed, with and without background radiation. The resulting multiple-input/multiple-output channel has the potential for combating fading effects on turbulent optical channels, for which both log-normal and Rayleigh-fading models are treated. Our focus is upon symbol error probability for uncoded transmission, and on capacity for coded transmission. Full spatial diversity is obtained naturally in this application.

Coherent optical communication using polarization multiple-input-multiple-output

Abstract: Polarization-division multiplexed (PDM) optical signals can potentially be demultiplexed by coherent detection and digital signal processing without using optical dynamic polarization control at the receiver. In this paper, we show that optical communications using PDM is analogous to wireless communications using multiple-input-multiple-output (MIMO) antennae and thus algorithms for channel estimation in wireless MIMO can be ready applied to optical polarization MIMO (PMIMO). Combined with frequency offset and phase estimation algorithms, simulations show that PDM quadrature phase-shift keying signals can be coherently detected by the proposed scheme using commercial semiconductor lasers while no optical phase locking and polarization control are required. This analogy further suggests the potential application of space-time coding in wireless communications to optical polarization MIMO systems and relates the problem of polarization-mode dispersion in fiber transmission to the multi-path propagation in wireless communications.

Underwater optical communications systems. Part 2: basic design considerations

Abstract: Acoustic systems may provide suitable underwater communications because sound propagates well in water. However, the maximum data transmission rates of these systems in shallow littoral waters are ~10 kilobits per second (kbps) which may be achieved only at ranges of less than 100 m. Although underwater (u/w) wireless optical communications systems can have even shorter ranges due to greater attenuation of light propagating through water, they may provide higher bandwidth (up to several hundred kbps) communications as well as covertness. To exploit these potential advantages, we consider the basic design issues for u/w optical communications systems in this paper. In addition to the basic physics of u/w optical communications with environmental noise, we consider system performance with some state-of-the-art commercial off-the-shelf (COTS) components, which have promise for placing u/w optical communications systems in a small package with low power consumption and weight. We discuss light sources which show promise for u/w optical transmitters such as laser diodes (LDs) and light emitting diodes (LEDs). Laser diodes with their output frequency shifted into the 500- to 650-nm range can emit more energy per pulse than LEDs but are more expensive. Currently, LEDs emit substantial amounts of light and are typically very inexpensive. Also, COTS photodiodes can be used as detectors which can respond to pulses several nanoseconds wide. Transmitter broadcast angles and detector fields of view (FOVs) with pointing considerations are discussed. If the transmitter broadcast angle and the detector FOV are both narrow, the signal-to-noise ratio (SNR) of the received pulse is higher but the pointing accuracy of transmitter and receiver is critical. If, however, the transmitter broadcast angle and/or the detector FOV is wide, pointing is less critical but SNR is lower and some covertness may be lost. The propagation of the transmitted light in various clear oceanic and turbid coastal water types is considered with range estimates for some COTS light sources and detectors. We also consider the effects of environmental noise such as background solar radiation, which typically limits performance of these systems

Optical communication device provided with a reflector and method for forming a reflector in an optical communication device

Abstract: An optical communication device of the invention includes a reflector for reflecting the light that has reached one end surface of a waveguide chip to turn the optical path of the light The reflector includes a transparent thin film layer formed on one end surface of the waveguide chip by using a material to which a metal that forms an intermetallic compound or the like with Au is added to a substance that is transparent to the light that propagates through the waveguide, as well as an Au thin film layer formed on the front surface of the transparent thin film layer This allows formation of a reflector having an Au thin film layer as a reflecting surface in an optical medium with high adhesion strength Thus, an optical communication device can be provided having a high reliability with little loss

Trends in satellite communications and the role of optical free-space communications [Invited]

Abstract: Feature Issue on Optical Wireless Communications (OWC) The communication needs of Earth observation satellites is steadily increasing. Within a few years, the data rate of such satellites will exceed 1 Gbit/s, the angular resolution of sensors will be less than 1 μrad, and the memory size of onboard data recorders will be beyond 1 Tbyte. Compared with radio frequency links, optical communications in space offers various advantages, such as smaller and lighter equipment, higher data rates, limited risk of interference with other communications systems, and the effective use of frequency resources. This paper describes and compares the major features of radio and optical frequency communications systems in space and predicts the needs of future satellite communications.

Self-powered communications link for smart circuit breakers

Abstract: A circuit breaker communication system is provided including a circuit breaker, an electronic module to record and store data associated with the operation of the circuit breaker, and a portable communication unit for downloading the stored data. The portable communication unit includes a power source, a display unit, a processor unit, a communication interface, and a keypad. Also, a cable is provided for interconnecting the portable communication unit to the electronic module. The cable is configured to carry a data signal between the portable communication unit and the electronic module, and to conduct power to the electronic module from the power source. The system may include an optical communication link to transmit to or receive signals from the circuit breaker, a transmitter to transmit data to the optical communication link; and a receiver to receive signals from the optical communication link.

Optical transmitting apparatus, optical receiving apparatus, and optical communication system comprising them

Abstract: A phase shift unit provides a prescribed phase difference (π/2, for example) between a pair of optical signals transmitted via a pair of arms constituting a data modulation unit. A low-frequency signal f 0 is superimposed on one of the optical signals. A signal of which phase is shifted by π/2 from the low-frequency signal f 0 is superimposed on the other optical signal. A pair of the optical signals is coupled, and a part of which is converted into an electrical signal by a photodiode. 2f 0 component contained in the electrical signal is extracted. Bias voltage provided to the phase shift unit is controlled by feedback control so that the 2f 0 component becomes the minimum.

Novel hollow optical fibers and their applications in photonic devices for optical communications

Abstract: Novel photonic devices based on a new type of waveguide, hollow optical fibers (HOF), are described. Utilizing unique three layered structure of HOF, the central air hole, germanosilicate ring core, and silica cladding along with its adiabatic mode transformation capability we demonstrated versatile applications in short-haul, long-haul optical communications, and tunable wavelength selective devices. Detailed design parameters, fabrication arts of the fibers, and operation principles of the devices are discussed.

Long optically controlled delays in optical fibers.

Abstract: Optically controlled delay lines in optical fibers are demonstrated by use of the group-velocity control of signal pulses based on stimulated Brillouin scattering. We achieve continuous time delay within the range of 150 ns, much larger than the width of the 40 ns signal pulse, using cascaded fiber segments joined by unidirectional optical attenuators. In the meantime, we also observe a large amount of pulse broadening, which agrees well with a theoretical prediction based on linear theory.

Achievable information rates for high-speed long-haul optical transmission

Abstract: There have been numerous attempts to determine the channel capacity of a nonlinear fiber-optic communication channel. The main approach was to consider amplified spontaneous emission (ASE) noise as a predominant effect and to observe the fiber nonlinearities as the perturbation of a linear case or as the multiplicative noise. In this paper, the achievable information rates for high-speed optical transmission (40 Gb/s and above) are calculated using the finite-state-machine approach. In calculations, the combined effect of ASE noise, Kerr nonlinearity [self-phase modulation (SPM), intrachannel four-wave mixing (IFWM), intrachannel cross-phase modulation (IXPM)], stimulated Raman scattering (SRS), chromatic dispersion, and (optical/electrical) filtering is taken into account.

Wireless optical communications via diversity reception and optical preamplification

Abstract: Atmospheric optical communication systems that use optical preamplifiers and diversity reception are addressed. The particular diversity techniques that are investigated include aperture averaging, linear combining, and adaptive optics. On-off keying and binary pulse position modulation are considered. The effects of atmospheric turbulence, background light, source extinction ratio, amplified spontaneous emission, and receiver thermal noise are studied in the context of a semiclassical photon-counting approach. Numerical results are presented using a conditional Gaussian approximation method. By this method, we can measure the power penalty incurred under various operating conditions as well as the link margin improvement due to optical preamplification and diversity reception.

A 3-Gb/s optical detector in standard CMOS for 850-nm optical communication

Abstract: This paper presents a monolithic optical detector, consisting of an integrated photodiode and a preamplifier in a standard 0.18-/spl mu/m CMOS technology. A data rate of 3 Gb/s at BER <10/sup -11/ was achieved for /spl lambda/=850 nm with 25-/spl mu/W peak-peak optical power. This data rate is more than four times than that of current state-of-the-art optical detectors in standard CMOS reported so far. High-speed operation is achieved without reducing circuit responsivity by using an inherently robust analog equalizer that compensates (in gain and phase) for the photodiode roll-off over more than three decades. The presented solution is applicable to various photodiode structures, wavelengths, and CMOS generations.

Measurement of eye diagrams and constellation diagrams of optical sources using linear optics and waveguide technology

Abstract: We demonstrate the characterization of optical sources with high sensitivity, high temporal resolution, and phase sensitivity using linear optical sampling. Eye diagrams and constellation diagrams are reconstructed using the interference of the source under test with a train of sampling pulses. This concept is implemented using a waveguide optical hybrid, which splits and recombines the sources and adjusts the phase between the recombined signals to provide optimal detection. This diagnostic is used to characterize on-off keyed (OOK) waveforms at rates up to 640 Gb/s and various phase-shift keyed (PSK) signals at 10 and 40 Gb/s.

Integrated transceivers for optical wireless communications

Abstract: Line-of-sight free-space optical links can provide extremely high bandwidth communications, but this usually requires that transmitter and receiver are precisely aligned. In order to allow terminals to be mobile, links must be able to track users within their field of view so that the link is maintained. There are various means to do this, but all require complex subsystems with a number of different optical, optoelectronic, and electrical components. A solid-state tracking architecture is introduced and a seven-channel tracking system demonstration described. The system is designed to operate at 155 Mb/s and is, to the best of our knowledge, the first that uses an integrated approach. Arrays of novel resonant cavity LED (RCLED) emitters that operate at 980 nm are used as sources. These are flip-chip bonded to arrays of CMOS driver circuits and integrated with the necessary transmitter optics. The receiver uses a back-illuminated detector array flip-chip bonded to arrays of custom CMOS receivers. All these components are custom and have performance substantially better than nonoptimized commercially available components. In the paper, the design and fabrication of the optics, optoelectronics, and electronics required for this is described. Successful operation of all the subsystems is detailed, together with results from an initial link demonstration.

Compensation for multimode fiber dispersion by adaptive optics.

Abstract: Adaptive optics is used to compensate for modal dispersion in digital transmission through multimode fiber (MMF). At the transmitter, a spatial light modulator (SLM) controls the launched field pattern. An estimate of intersymbol interference (ISI) caused by modal dispersion is formed at the receiver and fed back to the transmitter, where the SLM is adjusted to minimize ISI. Error-free transmission of 10 Gbit∕s non-return-to-zero signals through standard 50 μm graded-index MMFs up to 11.1 km long is demonstrated. It is shown that a single SLM can compensate for modal dispersion across a 600 GHz bandwidth.

Spectrally efficient optical CDMA using coherent phase-frequency coding

Abstract: We demonstrate feasibility of a spectrally efficient wavelength-division-multiplexing-compatible optical code-division multiple-access system using 16 phase-locked laser lines within an 80-GHz tunable window as frequency bins and an ultrahigh frequency resolution spectral phase encoder-decoder. Coding and decoding using binary [0, /spl pi/] phase chips were demonstrated for four users at 2.5 Gb/s, and a single coded signal was separated from four copropagating signals, with bit-error rate <10/sup -9/.

Ultimate channel capacity of free-space optical communications (Invited)

Abstract: Feature Issue on Optical Wireless Communications (OWC) The ultimate classical information capacity of multiple-spatial-mode, wideband optical communications in vacuum between soft-aperture transmit and receive pupils is considered. The ultimate capacity is shown to be achieved by coherent-state encoding and joint measurements over entire code words. This capacity is compared with the capacities realized with the same encoding and homodyne or heterodyne detection, which are single-channel-use measurements. Realistic background spectral-radiance values are used to obtain tight bounds on the capacity of single-spatial-mode, narrowband 1.55 μm wavelength free-space communications in the presence of background light.

All-optical multichannel 2R regeneration in a fiber-based device

Abstract: We propose the design of an all-optical 2R regenerator capable of handling multiple wavelength-division-multiplexed channels simultaneously. It extends the known concept of off-center filtering of self-phase-modulation-broadened signal spectra. The novel feature of the proposed device is a dispersion map that strongly suppresses interchannel impairments. The map employs several sections of nonlinear fiber with high normal dispersion, separated by dispersion compensators with spectrally periodic group delay. The results of our numerical simulations indicate the feasibility of such a multichannel regenerator.

Atmospheric compensation with a speckle beacon in strong scintillation conditions: directed energy and laser communication applications

Abstract: Wavefront control experiments in strong scintillation conditions (scintillation index, ≃1) over a 2.33 km, near-horizontal, atmospheric propagation path are presented. The adaptive-optics system used comprises a tracking and a fast-beam-steering mirror as well as a 132-actuator, microelectromechanical-system, piston-type deformable mirror with a VLSI controller that implements stochastic parallel gradient descent control optimization of a system performance metric. The experiments demonstrate mitigation of atmospheric distortions with a speckle beacon typical for directed energy and free-space laser communication applications.

Experimental demonstration of 10-gb/s data format conversions between NRZ and RZ using SOA-loop-mirror

Abstract: This paper describes the demonstration of a simple all-optical data format conversion scheme between return-to-zero (RZ) and nonreturn-to-zero (NRZ) that employs a semiconductor optical amplifier (SOA) in a nonlinear optical loop mirror. The format conversion has been performed between the most widely used data formats-NRZ and RZ formats. The format conversion scheme is based on gain variation by an intensity-dependent phase change in an SOA-loop mirror. The input data stream acts as a control signal that induces the phase differences between clockwise- and counterclockwise-propagating data inside an SOA-loop mirror. It is possible to change the data format of the output data stream by controlling the phase differences of the clockwise and counterclockwise pulse in an SOA-loop mirror appropriately. For the converted NRZ data from RZ data, 10-Gb/s error-free transmission up to 78 km over standard single-mode fiber has been obtained. By comparing the conventional NRZ transmission with the Mach-Zehnder modulation scheme, the proposed RZ-to-NRZ conversion shows an improved transmission performance. The NRZ-to-RZ conversion has clear eye openings up to 78 km. On the contrary, the conventional RZ binary data from a mode-locked laser has a nearly closed eye even at 52 km. The converted RZ data has a 2-dB conversion power margin to the injected NRZ data, which indicates an increase in the receiver sensitivity due to the signal format conversion. The improved transmission distance of the converted RZ signal is due to the duobinary coding effect of the SOA-loop mirror. The SOA has the possibility of high-speed operation over 40 Gb/s, and the SOA-loop mirror has the capabilities of format and wavelength conversions. Therefore, the SOA-loop mirror can be a universal building block in future all-optical networks. In addition, the proposed format conversion scheme can serve as an important format converter between the ultrafast optical-time-division-multiplexed networks and the lower line-rate wavelength-division-multiplexed networks.

10Gbit/s Multimode Fiber Link using Power-Efficient Orthogonal-Frequency-Division Multiplexing.

Abstract: Orthogonal Frequency Division Multiplexing (OFDM) can provide electronic dispersion compensation of optical paths. However, it requires a high bias to convert bipolar electrical signals to unipolar optical signals, so is inefficient in optical power for a given electrical signal to noise ratio. We present a novel method of transmitting OFDM signals over multimode fibers that increases electrical SNR by 7 dB for a given optical power. Using simulations, we show a 1.8 dB sensitivity benefit over 10 Gbit/s NRZ (Non-Return to Zero) and demonstrate compensation of inter-modal dispersion in a 300-m multimode fiber that cannot support NRZ.

Aperture averaging for optimizing receiver design and system performance on free-space optical communication links

Abstract: Feature Issue on Optical Wireless Communications (OWC) We have developed a flexible, empirical approach for optimizing the design of free-space optical communication links by using multiframe image analysis of received intensity scintillation patterns. This is a versatile way to perform aperture-averaging analysis. A high-performance digital camera with a frame-grabbing computer interface is used to capture received intensity distributions of a He-Ne laser beam propagating in weak and intermediate turbulence conditions. The aperture-averaging results demonstrate the expected reduction in intensity fluctuations due to increasing the receiver aperture diameter for various strengths of turbulence. Aperture averaging improves the bit error rate.

Radio-over-fibre technology for broadband wireless communication systems

Abstract: Wireless coverage of the end-user domain, be it outdoors or indoors (in-building), is poised to become an essential part of broadband communication networks. In order to offer integrated broadband services (combining voice, data, video, multimedia services, and new value added services), these systems will need to offer higher data transmission capacities well beyond the present-day standards of wireless systems. Wireless LAN (IEEE802.11a/b/g) offering up-to 54 Mbps and operating at 2.4 GHz and 5 GHz, and 3G mobile networks (IMT2000/UMTS) offering up-to 2 Mbps and operating around 2 GHz, are some of today’s main wireless standards. IEEE802.16 or WiMAX is another recent standard aiming to bridge the last mile through mobile and fixed wireless access to the end user at frequencies between 2 – 66 GHz. The need for increased capacity per unit area leads to higher operating frequencies (above 6 GHz) and smaller radio cells, especially in in-door applications where the high operating frequencies encounter tremendously high losses through the building walls. To reduce the system installation and maintenance costs of such systems, it is imperative to make the radio antenna units as simple as possible. This may be achieved by consolidating signal processing functions at a centralised headend, through radio-over-fibre technology. The research in this thesis focussed on the feasibility of using both single-mode and multimode fibres to distribute high-frequency microwave signals to simplified remote radio antenna units. An alternative radio-over-fibre technique, termed Optical Frequency Multiplication (OFM) has been investigated. OFM entails the periodic filtering of a swept optical signal at the headend followed by photodetection at the radio access unit. A low sweep frequency (e.g. 3 GHz) is used. After photodetection at the remote radio access unit, high-frequency (>21 GHz) harmonic components of the sweep signal are generated. The desired microwave signal is selected by means of bandpass filtering, amplified, and radiated by the antenna. Modulated microwave carriers are generated by intensity modulating the frequency-swept optical signal. Through modelling, simulations, and extensive experiments, the behaviour and performance of a radio-over-fibre downlink employing OFM was investigated. Simulation and comprehensive experimental results showed that OFM can be used to generate pure high-frequency microwave signals with very narrow linewidth and low SSB phase noise. This is because in the OFM process laser phase noise is inherently suppressed. The low-phase noise capability of OFM enables it to support the delivery of carriers modulated not only by the simple ASK data format, but also by complex multilevel modulation formats such as BPSK, QPSK, and x-level QAM. Multicarrier signals such as Subcarrier Multiplexed signals, and OFDM signals used in wireless LANs are also supported. Low Error Vector Magnitudes (below 5%) were obtained for x-QAM modulation formats, including 64-QAM. BER measurements showed a modal dispersion penalty of about 1 dB for a 4.4 km MMF link under restricted launch condition. It was established that OFM is chromatic dispersion tolerant and can support more than 10 times longer single-mode fibre transmission links (exceeding 50 km) than IMDD systems, which suffer from the chromatic-dispersion-induced amplitude suppression. OFM also enables the delivery of microwave carriers exceeding the modal bandwidth of MMFs, by using the higher transmission passbands of the fibre response. Silica glass MMF links of more than 4 km are feasible. The maximum link length, which can be bridged with Polymer Optical Fibre (POF) is significantly shorter, owing to its higher attenuation values. Thus POF may be more attractive for in-building applications where link lengths of 500m are often sufficient. Several different implementations of the Mach Zehnder Interferometer, and the Fabry Perot Interferometer filters were considered to determine their simplicity, performance, and applicability within the end-user environment. It was established that the wavelength of the optical FM source needs to be carefully aligned to the characteristics of the periodic optical filter. Therefore, it is preferred that both the source and the filter are co-located. This makes it easier to employ electronic tuning control of the filter (e.g. a fibre Fabry Perot Interferometer), so as to automatically track the alignment with the optical source, resulting in remarkable improvement of the OFM system stability. The ability to achieve high frequency multiplication factors, good phase noise performance, the support for all modulation formats, and the ability to operate on both single-mode and MMFs, all make OFM ideal for use in high-frequency (>5 GHz) broadband wireless system applications.

Short Range Underwater Optical Communication Links

Abstract: CHANCEY, MARK ALAN. Short Range Underwater Optical Communication Links. (Under the direction of Dr. John F. Muth) The future tactical ocean environment will be increasingly complicated. In addition to traditional communication links there will be a proliferation of unmanned vehicles in space, in the air, on the surface, and underwater. To effectively utilize these systems improvements in underwater communication systems are needed. Since radio waves do not propagate in sea water, and acoustic communication systems are relatively low bandwidth the possibility of high speed underwater optical communication systems are considered. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this thesis is to understand how to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. The secondary focus of the thesis was to construct LED based optical communication systems. This required understanding the behavior of Gallium Nitride LEDs operated under intense electrical pulsing conditions. An optical FM wireless system was constructed for transmitting speech. An LED based Ethernet compatible digital communications system that was capable of operating at 10 Mbps was also constructed and packaged for underwater operation. SHORT RANGE UNDERWATER OPTICAL