The preferred method for ultra-high bandwidth in communications has been the optical. This is mainly due to the demand for more bandwidth, which the 1300-1650nm band of light offers in an optically transparent medium, air or fiber. Although fiber technology has been advanced to produce a wide range of fiber types to meet specific needs, fiber deployment requires long-term planning and installation that culminate to long delays till full service is established. There are applications that demand high-data rate connectivity urgently, either because of natural emergencies or because the private sector cannot wait for the long planning and deployment. Thus, an alternative communication method is free space optical. This method uses a laser beam that is directed from a building to another to establish full-duplex point-to-point high-data rate connectivity. However, this technology has been limited to a point-to-point topology. In this paper we present a free space optical integrated node design suitable for mesh and/or ring topology with full connectivity. We also discuss engineering aspects, traffic management, fault management and security issues.

A compact and mobile distributed sensing system which can monitor contaminant content in the ocean and initiate an alert system when high contaminant levels are detected would be useful in the surveillance of ports and harbors. Oceanic data acquisition takes many forms, but miniature and low cost platforms are not available for widespread monitoring and surveillance tasks. In this paper we present a novel sensing system, adapted from a similar concept designed for atmospheric probing and evaluate its feasibility in the ocean environment. Both the probing task itself and the data communication from the sensor nodes to the base station are based on free space optics. We also develop a probabilistic model of multi-access interference in a system using spectral diversity, which may be applicable in many distributed sensor multihop networks.

Anti-correlation polarization dynamics in vertical-cavity surface-emitting lasers with long external cavity has been studied experimentally. The correlations are considered in the time domain and in the spectral domain. In the time domain, with fixed optical feedback strength, the minimum cross-correlation coefficient exhibits exponential decay with the bias current. With fixed bias current, strong anti-correlation can be achieved at a certain feedback ratio. In the frequency domain, strong anti-correlation is obtained at frequencies lower than 1 GHz for all bias currents. Near the relaxation oscillation frequency good correlation is found at higher bias current but poor correlation is found at lower bias currents.

Free-space optical communications have distinct advantages over conventional RF and microwave systems by virtue of their high carrier frequencies that permit high modulation bandwidth, enhanced security, freedom from interference, and low powered. However, the turbulent atmosphere causes phase variations along the path that are manifested in intensity variations (scintillation) and high beam divergence. These variations are a noise source that reduces the ability of the receiver to determine the information contained in the modulation. For many years, the emphasis throughout this area has been on elucidating those implications of the atmospheric propagation problem that bear on the design and performance of optical communication systems. In this work, it is our intention to elucidate how the addition of adaptive optics to the transmitter or receiver can reduce the effects of atmospheric propagation and, in so doing, to quantify the improvement on the performance of optical communications systems regarding incoherent detection. Adaptive optics offers the potential for overcoming these limitations by adaptive tracking of the beam and correction of atmospherically-induced aberrations.

Atmospheric turbulence can significantly degrade the performance of free-space optical communication links. Beam wander, intensity scintillations, beam broadening and angle of arrival fluctuations give rise to signal fading with communication channel drop-outs. In this work the performance of the potentially beneficial combination of a retro reflective link operating at 1535 nm in conjunction with a single-photon quantum key distribution link at 850 nm was studied. The general idea is that the weak light beam of the quantum channel can be steered and controlled by the 1535 nm signal. A dual-channel laser unit, with output beams at 850 and 1535 nm, was used to characterise turbulence effects along an atmospheric path close above ground. The laser beams were retro reflected from a corner cube and detected in the same laser unit. The 1535 nm beam was monitored with a position sensitive detector while the 850 nm beam (low intensity channel) was aligned with the 1535 nm beam and detected separately but along the same optical axis of the laser unit. Effects of beam wander, angular fluctuations and intensity scintillations were studied experimentally at different turbulence strengths and weather conditions. The experimental results are discussed with respect to theoretical models.

The performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. In this work, fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gamma-gamma distributions. The probability of fade and expected number of fades per second were calculated from 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 calculate the parameters for the theoretical distributions. A new integral expression for the expected number of fades based on the gamma-gamma distribution was developed and shown to make a significant difference compared to the existing approximation.

Free space optical communications using 9-10 fs pulses was investigated through aerosol clouds approaching 10⁴ to 10⁵ particles per cm³ in a 15 cm long tube containing aerosol particles 4-5 μm in diameter. This small size is representative of the most difficult situation for the transmission of light through the atmosphere. The ultra fast pulse transmission were compared to continuous wave (cw) transmission through the exact same aerosols clouds and compared to the ultra short pulses. Results indicate that there was increased transmission for the ultra short laser pulses, but was not as high as expected. The results now can be explained by a recent paper demonstrating that deviation from the Beer Lambert law does not occur until the ultra short pulse transverses a longer path length in water. Results will be presented on the pulse dispersion in water and glass.

Retro-communication by the joint use of light modulators and retro-reflecting devices has been proposed and demonstrated as a promising technique to further extend the use of free-space optical communication (FSO) to mobile communication. We have developed and demonstrated high-speed surface-normal GaAs-based and InP-based multiple quantum well electroabsorption modulators and arrays operating at different near-infrared wavelengths for FSO link applications. In this paper we report on the structure and device design issues, and trade-offs when GaAs-based or InP-based modulators are needed in the FSO links. In particular, the modulator structure properties, the device fabrication techniques and the device static and dynamic performance are investigated and compared.

Recent field trials have shown that modulated retro-reflective (MRR) optical communications is a potentially feasible technique for applications demanding high data rates. Data rates over 10 Mbit/s has been demonstrated with Multiple Quantum Well (MQW) modulators in experimental MRR systems. An MQW-based MRR has a variable reflectance and information can thus be transmitted to the receiver by modulating the intensity of the reflected signal. However, current experimental systems normally use binary modulation (e.g. on-off keying) and the data rate is then strictly limited by the modulation speed of the retro-reflector. Instead, by employing a multiple-level modulation scheme the data rate can be increased substantially. Herein, we discuss if the use of different signal processing techniques, commonly used in radio communication systems, may improve the robustness and capacity of MRR free-space optical communication links. Techniques of interest are mainly error-correcting codes, link adaptation and high-level modulation schemes. Furthermore, we apply some of these techniques on measured channel data that has been collected in recent field trials. Through simulations we demonstrate the potential gains that can be achieved through the use of link adaptation and multiple-level modulation. Finally, a brief comparison with competing radio techniques is given.

In this work we report on a specially optimized type-I InAsSb/InAsSbP double heterostructure (DH) ridge laser grown by liquid phase epitaxy (LPE). To remove residual impurities and reduce Shockley-Read recombination, the active region was purified using a Gd gettering technique. In addition free carrier absorption loss was minimized by the introduction of two undoped quaternary layers with the same composition of the cladding layers either side of the active region. The inserted layers also helped alleviate inter-diffusion of unwanted dopants towards the active region during or after growth and reduced current leakage of the device. The diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 μm at 170 K with a threshold current density as low as 118 A/cm² at 85 K. Compared to the conventional 3-layer DH laser, the optimized 5-layer structure with reduced optical loss can raise the maximum lasing temperature by 95 K to ~210 K.

The paper deals with the analysis of receivers for free-space optical communication systems. The performance of high impedance and transimpedance configurations of front-end amplifiers in combination with an avalanche photodiode and a PIN photodiode are compared. The theoretical sensitivity analysis of optical fiber receivers has been presented in many previous works. But free-space optical systems make somewhat different demands on the receivers. One of them is the relatively large active area of photodiode which is necessary for an optimal coupling to the receiver lens. Large active area causes a large junction capacitance that reduces the bandwidth and increases preamplifier noise as well as photodiode dark current and consequently the photodiode shot noise. These effects decrease the receiver sensitivity, dynamic range and allowable bit rate. The dark current also depends on the photodiode type and the technology and material used. Similarly, bipolar and FET transistors used in the high impedance and transimpedance preamplifiers do not provide the same resulting receiver sensitivity and bandwidth due to their unequal noise and frequency characteristics. Another demand made on the free-space optical receiver is the high dynamic range necessary to limit the influence of varying attenuation in the atmosphere. It can be partially improved by incorporating the automatic gain control loop.

Coding data bits in the phase or polarisation state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fibre and free-space quantum cryptography apparatus used for secure exchange of keys. To date free space key exchange has aimed at long range with 144km range achieved and future experiments aiming to extend this range to 1000km exchanging keys with low earth orbit satellites. At the other end of the spectrum we are developing low cost hand held systems. These systems could be an effective way for the user to generate a store of secrets shared with a central repository. These secrets can then be used up to protect a wide variety of sensitive classical communications. Examples include on-line PIN protection for consumer transactions and password protection in secure access schemes.

We present two types of photon sources designed for secure quantum communication, e.g. for quantum cryptography. Both types are based on the creation of photon pairs by spontaneous parametric downconversion in nonlinear crystals. The first is a heralded single photon source and the second is a source of polarization-entangled photon pairs. For the heralded single photon source the detection of one of the photons of a downconversion pair is used as a trigger to announce the presence of the other: the single photon. The source is characterized by a highly sub-Poisson photon number statistics making it very suitable for use in quantum cryptography protocols using single photonic qubits to create correlated information between a sender and a receiver. The entanglement source instead uses the inherent non-classical correlations between entangled qubits. We also present a hybrid-encoding where the sender uses polarization to encode information while the receiver uses time-bins. Both sources create photons with highly non-degenerate wavelengths of 810 nm and 1550 nm, taking advantage of the efficient detectors at near-infrared and the low transmission loss of optical fibers at telecommunication wavelengths.

We report on the experimental implementation of a BB84-type quantum key distribution protocol over a 144 km free-space link using weak coherent laser pulses. The security was assured by employing decoy state analysis, and optimization of the link transmission was achieved with bi-directional active telescope tracking. This enabled us to distribute a secure key at a rate of 11 bits/s at an attenuation of about 35dB. Utilizing a simple transmitter setup and an optical ground station capable of tracking spacecraft in low earth orbit, this outdoor experiment demonstrates the feasibility of global key distribution via satellites.

Unconditionally secure message authentication is an important part of Quantum Cryptography (QC).We analyze security effects of using a key obtained from QC in later rounds of QC. It has been determined earlier that partial knowledge of the key in itself does not incur a security problem. However, by accessing the quantum channel used in QC, the attacker can change the message to be authenticated. This, together with partial knowledge of the key does incur a security weakness of the authentication. We suggest a simple solution to this problem, and stress usage of this or an equivalent extra security measure in QC.

Quantum key distribution (QKD)¹ is the first method of quantum information science that will find its way into our everyday life. It employs fundamental laws of quantum physics to ensure provably secure symmetric key generation between two parties. The key can then be used to encrypt and decrypt sensitive data with unconditional security. Here, we report on a free space QKD implementation over a distance of 480 m using strongly attenuated laser pulses. It is designed to work continuously without human interaction. Until now, it produces quantum keys unattended at night for more than 12 hours with a sifted key rate of more than 50 kbit/s on average and a quantum bit error rate between 3% and 5%.

A novel low jitter optical pulse source for applications including optical sampling has been modelled and experimentally verified using commercially available photonic components. Dispersion and non-linear fibre effects were utilised to compress a periodic optical waveform to generate pulses of the order of 10-15 picoseconds, via soliton-effect compression. Modelling of variable dispersion fibre indicates that future work will enable pulse compression to sub-picosecond durations. Attractive features of this new pulse source include electronically tuneable repetition rates over a continuous range at frequencies above ~1.5 GHz, ultra-short pulse duration (10-15 ps now, 100's fs planned), and low timing jitter as consistently measured by both harmonic analysis and single-sideband (SSB) phase noise measurements. In addition, the system is inherently robust to thermal effects unlike some low jitter pulse source alternatives, and does not require precise optical alignment. Timing jitter analysis reveals that the optical pulse timing jitter is currently limited by the mid-range specification microwave source used to create the initial periodic waveform. In order to overcome stimulated Brillouin scattering at high launch powers into the compression fibre, phase modulation was applied to the pulse train, and the timing jitter implications of this are discussed. It is believed that this is the first time that detailed timing jitter analysis has been performed on a soliton-effect compression scheme.

We have developed 1.3 μm quantum dots (Qdot) using a dot in a well (DWELL) structure based on GaAs and 1.55 μm quantum dash (Qdash) based on InP Fabry-Perot lasers using a ridge waveguide operating in continuous wave at room temperature. The quantum dot lasers have demonstrated high power of 135 mW per facet and 50 mW per facet for the quantum dash devices. We have obtained very low relative intensity noise (RIN) with a nearly flat spectrum, around -159 dB/Hz ± 2 dB/Hz within 0.1-10 GHz range for the quantum dots and -160 dB/Hz ± 2 dB/Hz over a wide bandwidth from 50 MHz to 18 GHz for the quantum dash lasers. Recent experimental results are presented and analysed especially those relating to the noise performances and reliability tests to demonstrate the suitability of these new devices for microwave optical links.

Because electro-optic switches are key elements for the synthesis of large bandwidth, high dynamic, true time delay optically controlled microwave antennas, we designed, fabricated and characterized a new digital optical switch grown on InP substrate, and based on carrier-induced effects. The switching time is short (3ns) and there is no added noise on the optical signal. By introducing this switch in a microwave optical link at 1GHz frequency, we demonstrate that a 72dB microwave crosstalk (more than 36dB on the optical signal) can be achieved. The low current consumption of 60mA makes our device suitable for the optical control of microwave antennas.

An opto-electronic transient waveform digitiser is detailed which combines the massive bandwidth of commercially available optical components with the high resolution sampling capabilities of state-of-the-art electronic analogue to digital converters (ADCs). Core to the success of a 64 GSa/s sampling rate has been the development of an automated temperature stabilised fibre delay line structure. This generates multiple replicas of short incoming pulses, which are then digitised using the Vernier sampling technique. The successful temperature stabilisation of the optical sub-components now offers the potential for an environmentally rugged fibre based system, with small system footprint, and low power requirements through the use of a single electronic ADC and compact diode laser technology. System modelling indicates that the fibre delay line architecture performance could currently support very high sample rates, with the total system sample rate scaling directly with future electronic ADC improvements.

Transient phenomena contain rich intrinsic features as shown in the famous Edgerton's photo of a bullet piercing through an apple with the help of a stroboscope. Although a photo is able to freeze transient phenomena, it still lacks the ability to reveal intrinsic micro motions of constituents. The advancement of optical fibers has changed the above scenario. An RF transient signal and its reference can be fed into optical fiber recirculation loops, which are able to regenerate numerous replicas of these signals. The micro motions captured by the transient signal can then be deciphered through the repeated analysis of its replicas. A new instrument which is capable of accomplishing the above is known as an interferoceiver. It will have a broad impact on radar technology as well as on remote sensing and passive identification. The present talk will discuss its capabilities and experimental results.

The authors describe a 360 degree viewing display that can be viewed from any direction. This new viewing system has an illumination screen and a rotating table. The screen of a monitor rotates at a uniform speed. Then an observer can view a monitor screen at any position surrounding the round table. But the solid of revolution is formed when the image screen is rotated. Hence the angle of view is controlled by a slit in order that the screen faces an observer and he can view an only 2D image on the screen without a 3D solid image. In this paper, we also describe a geometrical analysis of the 360 degree viewing display.

A display system that does not require viewers to wear special glasses to see 3D images is useful technology. Conventional 2-views lenticular and parallax-barrier display systems have disadvantages such that horizontal resolution is reduced by half because each eye only sees half the pixels. We describe a method to generate orthogonal polarized parallax images and a thin stereoscopic 3D display with a polarized parallax barrier. The advantage of the polarized parallax barrier is that the resolution of presented parallax images is much greater than that possible with conventional displays. This compact display has liquid crystal display(LCD) panels and/or Electoro Luminescence(EL) devices to present stereo views and to control polarization. It has twice the 3D image resolution, because the image plane can multiplex images with horizontal and vertical polarization to display stereo views.

A display system requiring no special glasses is a useful technology for 3D images. The parallax barrier display system has superior characteristics, such as having a planar screen and a thin panel. However, conventional multi-view parallax barrier display system has disadvantages such that the resolution of each stereoscopic image is reduced only in the horizontal dimension. This paper describes the development of two models for displaying 4-views images. One is the 4-views display with a parallax polarizer barrier and the special backlight. In this display, the viewing points of parallax images can be arranged arbitrarily. The other is the stereoscopic display with the same 4-views using the combination of a parallax polarizer slit and cylindrical lenses. It is possible that the image resolution of each view is reduced both horizontally and vertically using this combination. These stereoscopic 3D displays can avoid the horizontal resolution problem of conventional system.

The laser diodes are efficient sources of optical radiation. The maximum optical peak power depends on the pulse duration of the driving current pulse - reducing the pulse duration the safety peak power is increased. The aim of the study is to elaborate a methodology to determine the safety operation area of the high power laser diodes designed for CW operation. Using this methodology, the designer of certain laser devices may exploit the maximum of the laser diode under pulsed operation.