Cat's eye quantum well modulating retroreflectors for free-space optical communications

Introduction Free-space optical communication (FSO) offers many potential advantages. These include: high bandwidth, security, non-interference, and a large amount of available spectrum. Many of these advantages are related to optics' short wavelength relative to radio-frequency (RF) or microwave systems. At optical wavelengths even a small transmit aperture can produce highly directional beams, and optical receivers can focus light down to very small spots. To make use of this high optical antenna gain very good pointing is required. Even a small telescope, with a 1 cm diameter, has a diffraction-limited beam divergence of less than 250 micro-radians in the near-infrared. Pointing with this accuracy is challenging, particularly if one or both ends of the link are in motion. Active FSO systems with high pointing accuracy are often large, heavy, and have high power consumption [1]. Such systems are also complex, which can lead to reliability issues. Despite these challenges, direct FSO links – those with active terminals on both ends – have many good applications. There are, however, other applications in which the two ends of the link have different payload and power capabilities. Some examples include: unattended sensors, small unmanned aerial vehicles (UAVs), and small, tele-operated robots. For these applications a modulating retro-reflector (MRR) may be an appropriate solution. Optical MRRs couple passive retro-reflectors with electro-optic modulators to allow long-range, free-space optical communication with a laser, and pointing/acquisition/tracking system, required on only one end of the link. As shown in Figure 13.1, a conventional free-space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a continuous-wave beam.

Cat’s eye lens make the laser beam retro-reflected exactly to the opposite direction of the incidence beam, called cat’s eye effect, which makes rapid acquiring, tracking and pointing of free space optical communication possible. Study the influence of cat’s eye effect to cat’s eye retro-reflector at large incidence angle is useful. This paper analyzed the process of how the incidence angle and focal shit affect effective receiving area, retro-reflected beam divergence angle, central deviation of cat’s eye retro-reflector at large incidence angle and cat’s eye effect factor using geometrical optics method, and presented the analytic expressions. Finally, numerical simulation was done to prove the correction of the study. The result shows that the efficiency receiving area of cat’s eye retro-reflector is mainly affected by incidence angle when the focal shift is positive, and it decreases rapidly when the incidence angle increases; the retro-reflected beam divergence and central deviation is mainly affected by focal shift, and within the effective receiving area, the central deviation is smaller than beam divergence in most time, which means the incidence beam can be received and retro-reflected to the other terminal in most time. The cat’s eye effect factor gain is affected by both incidence angle and focal shift.

High-speed modulating retro-reflectors with optical phase conjugation compensation

The paper discusses the atmospheric turbulence effect on modulating retro-reflector (MRR) optical communication systems with theoretical analysis and experimental verification. The transmission model is proposed by the phase screen method. Based on this model, the scintillation indices (SI) are calculated, and the aperture averaging effect of the MRR terminal is analyzed. Furthermore, a test bed with 506 m transmission link is established. In the experiment, the SI values of the MRR link and the direct link are obtained and compared. Experimental data agree well with the numerical simulation results and verify the theoretical model. The theoretical results are useful for analyzing the influence of turbulence on MRR systems.

To enhance the coordinates measuring accuracy of the multi-beam laser tracking 3D coordinates measuring system, measurement for the optical error of the cat's eye retro-reflector is necessary. For this purpose, measurement method for the optical error of the cat's eye retroreflector is proposed. The main sources of the optical error of cat's eye retro-reflector are analysed first, and associated experiment setup and measuring data processing method are described. Experiment results show that the maximum optical error of the measured cat's eye retro-reflector is approximately 4μm.

Performance analysis of modulating retro-reflector FSO communication systems over Málaga turbulence channels

A 1550-nm eye-safe, free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors. Tests are conducted under various atmospheric conditions over a time period of about a year. The experimental and theoretical link budgets are compared and statistical measurements of the effects of scintillation are collected.

The channel characteristics of modulating retro-reflector (MRR) free-space optical (FSO) link are studied by using wave-optics (WO) simulation method. The optical wave launched from the transceiver is reflected back in the same direction to the transceiver by the use of cube-corner reflector (CCR). This propagation process is divided into three parts: forward propagation through atmospheric turbulence, reflection of CCR, and backward propagation through the turbulence. For WO simulation under turbulence conditions, the split-step beam propagation method is implemented, and the influence of atmospheric turbulence along the propagation path is described by a set of random thin phase screens. The reflection effect of CCR is taken into account by using ray-tracing approach to calculate the intensity and phase of the optical field output from CCR. Integrating the reflection effect into WO simulations, we study the statistics of the intensity fluctuations in the forward, backward and double-pass links, and the correlation of fluctuations in the forward and backward links. The histograms of the normalized intensity for three links could be approximated by Gamma-Gamma (GG) distributions except for relatively large receiver apertures. There exists a relatively large correlation between the forward and backward links.

Free space optical (FSO) communication systems have attracted great attention due to their ability to achieve high data rates. To increase the coverage area, multi-antenna systems were proposed, which failed to achieve diversity gains due to limitations of the feasible number of antennas per terminal and turbulent induced fading. Cooperative communication can overcome these drawbacks and this paper reviews relay assisted communication. The performance of Amplify and forward (AF), Decode and Forward (DF) and selective relaying protocols are analyzed and compared based on the metrics of outage analysis, BER analysis and communicating distance(s).

Free Space Optical (FSO) communications is the only viable solution for creating a three-dimensional global communications grid of inter-connected ground and airborne nodes. The high amount of data exchange between satellites and ground stations demands enormous capacity that can not be provided by strictly regulated, scarce resources of the Radio Frequency (RF) spectrum. Free Space Optical (FSO) communications, on the other hand, has the potential for providing virtually unlimited bandwidth. Furthermore, due to the spatial confinement of laser beams, such links are very secure. In other words, security is guaranteed at the physical layer. However, the promised enormous data rates are only available under clear weather conditions, and atmospheric phenomena such as clouds, fog, and even turbulence can degrade the performance, dramatically. While scattering media such as clouds and aerosols cause pulse broadening in space and time, turbulence presents itself as scintillation and fading. Hence, to exploit the great potential of FSO at its best under all weather conditions, prudent measures must be taken in the design of transmitter and receiver. More specifically, multiple transmitters and receivers can be used to de-correlate the turbulence induced fading and to compensate for pulse attenuation and broadening caused by scattering. In this paper, Multiple-Input Multiple-Output (MIMO) transmitter and receiver designs in FSO communications are investigated and the achievable performance improvements are discussed.

Free-Space Optical communication technology is a potential solution to the last mile or broadband access problem. Conventional free-space optical (FSO) communication is over a single link between two nodes. We explore multi-channel FSO communication system using compact (a maximum of a Sq.Ft) 2-dimensional antennas with multiple communication links between them to achieve very high aggregate bandwidths (100's of Gbps). But, close packaging of optical channels on the arrays causes inter-channel interference, reducing per channel capacity. We model the error due to inter-channel interference for such arrays and estimate the channel capacity. We address the multi-channel interference issue by both array design and by employing optical orthogonal codes (OOCs) for free-space optical communications and show that we can achieve multi Gbps bandwidths using such arrays. Possible applications for such multi-channel FSO systems can be in multi-hop broadband access networks or mesh networks and in back haul, connecting wireless base stations.

In recent year the free-space optical communication (FSO) has become more interesting as its alternative to radio frequency communication. The links of Free-space optical (FSO) communication very sensitive for a lot of variability which make a real challenge for efficient, robust system design. In free space optic the information sends through the optical fiber as pluses, so a large dynamic range will provide, from designed systems in most cases. The FSO is a line-of-sight (LOS) technology that transmits a modulated beam of visible or infrared light through the atmosphere for broadband communications. The focus of this work is to investigate the feasibility and design of free-space optical (FSO) with millimeter waves for 5th generation. Consequently, the conducted analysis done by using optisystem software. As a result, the analysis generated based on Eye diagram, transmitted power, Q factor and distance.

In this thesis we report the studies of optical properties of ZnMnSe/ZnSeTe multiple quantum wells. Quite interesting results have been obtained from our studies which should be very useful for our understanding and application of these materials. Ⅰ. Novel properties of photoluminescence in type-II ZnMnSe/ZnSeTe multiple quantum wells The optical properties of type-II ZnMnSe/ZnSeTe multiple quantum wells were investigated by photoluminescence. From the photoluminescence (PL) spectra, a giant blueshift of the peak energy under moderate excitation power has been observed. It can be well interpreted in terms of the band-bending effect due to spatially separated photoexcited carriers in a type-II band alignment. In addition, a large optical anisotropy of ZnMnSe/ZnSeTe multiple quantum wells (MQWs) has been discovered. Based on the studies of the dependences of excitation intensity and temperature, we point out that the observed anisotropy is an inherent property of the type-II band alignment in ZnMnSe/ZnSeTe MQWs. Ⅱ. Thickness dependence of optical properties of ZnMnSe/ZnSeTe multiple quantum wells Photoluminescence spectra and Raman scattering were performed to study thickness dependence of optical properties of ZnMnSe/ZnSeTe multiple quantum wells. Strong photoluminescence associated with iso-electronic Te traps was observed. As the thickness of the ZnSeTe layer decreases to 1 nm the mechanism of luminescence changes. In Raman spectra, the LO and TO phonons of ZnSeTe layer were observed. A blueshift of optical phonon modes was observed as the thickness of the ZnSeTe decreases. This phenomenon can be well interpreted in terms of the compressive strain effect.

We review the occurrence of periodic or near periodic electric-field domains stimulated by bound-to-continuum transitions under the influence of infrared radiation in multiple uncoupled quantum wells. The conditions of formation of such domains and their stability are studied theoretically using the proposed phenomenological model.

III-nitride wide bandgap semiconductors have been at the center stage of the semiconductor R & D effort over the last decade. Photonic/electronic devices based on III-nitrides, including UV/blue/green/white LEDs, violet/blue LDs, UV detectors, and high power/temperature transistors, have reached high performance level. This talk will provide a brief overview on recent advances made by our group and others in the area of III-nitrides for energy and hydrogen generation. The presentation will concentrate on topics of material and device challenges of III-nitride solar cells and photoelectrochemical cells (PECs). The bandgap of InGaN expands from about 0.65 to 3.4 eV, which covers the entire solar spectrum. In principle, a multi-junction solar cell based on multiple layers of InGaN with different In-contents is capable of capturing the entire spectrum of the sunlight and can thus be highly efficient. To count for the relatively low materials quality for high In content InGaN alloys, InGaN/GaN multiple quantum wells (MQWs) have been adopted for the construction of nitride solar cells. We have grown, fabricated, and characterized InGaN based solar cells utilizing the InGaN/GaN MQWs p-i-n structures.1-3The fabricated solar cells exhibited open circuit voltage of about 2 V, a fill factor of about 60% and overall efficiency of about 3% under AM 1.5 irradiation. The performance of these nitride solar cells under concentrated sunlight has also been investigated. In additional to its bandgap match to the solar spectrum, the bandgap of InGaN alloys can also be engineered to optimally match with conditions necessary for direct hydrogen generation by water splitting using sunlight.4-6 The results on hydrogen gas generation via solar water splitting accomplished using n- and p-type InGaN epilayer as working electrodes will be discussed. Direct solar water splitting and hydrogen gas generation under zero bias has been realized using an InGaN/GaN multiple quantum well solar cell as a photoelectrochemical cell. Under the irradiation by a simulated sunlight (AM1.5G solar simulator), a 1.5% solar-to-fuel conversion efficiency has been achieved under zero bias, setting a new benchmark of employing III-nitrides as photoreactive electrodes for artificial photosynthesis. Time dependent hydrogen gas production photocurrent measured over a prolonged period revealed an excellent chemical stability of InGaN in aqueous solution. Our results indicate that InGaN is an excellent artificial photosynthesis material which is capable to provide usable clean fuel (hydrogen gas) with the sunlight being the only energy input. InGaN materials growth effort was supported by NSF (DMR-1206652) and the energy device effort was by DOE (FG02-09ER46552). Jiang & Lin are grateful to AT&T Foundation for the support of Ed & Linda Whitacre endowed chairs. R. Dahal, B. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN multiple quantum well solar cells with long operating wavelengths,” Appl. Phys. Lett. 94, 063505 (2009). R. Dahal, J. Li, K. Aryal, J. Y. Lin, and H. X. Jiang, "InGaN/GaN multiple quantum well concentrator solar cells," Appl. Phys. Lett. 97, 073115 (2010). B N. Pantha, J. Y. Lin, and H. X. Jiang, “III-nitride nanostructures for energy generation,” Proc. SPIE 7608, 76081I (2010). J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett. 93, 162107 (2008). K. Aryal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, "Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells," Appl. Phys. Lett. 96, 052110 (2010). R. Dahal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “Realizing InGaN monolithic solar-photoelectrochemical cells for artificial photosynthesis,” Appl. Phys. Lett. 104, 143901 (2014)