Francisco J. Lopez-Hernandez

Experimental optical code-division multiple access system for visible light communications

An optical code-division multiple access system (OCDMA) is presented for visible light communications (VLC). The OCDMA scheme is based on the recently proposed random optical codes (ROC), which do not present optimal correlation properties but are characterized by their ease of generation when large number of simultaneous users are considered. The synchronism mechanism when using ROCs is an important issue which is adequately addressed and solved by the designed system. An experimental prototype has been developed which allows us to transmit medium-quality audio data from several emitting sources simultaneously through different channels (using different codes), obtaining a practically error-free reception (imperceptible by the human ear) which perfectly agrees with the theoretical analysis.

Random optical codes used in optical networks

An analysis of a simple way to design generalised optical orthogonal codes to be used in optical code-division multiple-access systems is presented. They are random optical codes (ROCs). This novel family is especially suitable for optical networks with a large number of channels, low bit rate requirements, energy limitations and packed data. For example, sensor networks need these requirements. An application in an intra-spacecraft telecommand and telemeasurement (TC/TM) optical wireless network is also described. ROCs are selected randomly from all possible rearrangements. However, its probability of error is adequate in many useful parameter ranges. Moreover, ROCs present features not found in other families of codes such as huge cardinality and parameter adaptation to the system necessities. Here, the main contribution is to present a method to optimise the values of the parameters. Furthermore, the optimal weight of the codes is found, given two out of the other set of parameters, that is, the length, the number of users and the probability of error. In addition, a comparison between the length and the power consumption of ROCs and prime codes is also developed.

Random optical codes in an intra-satellite optical wireless network

We present a novel family of optical codes to be used in intra-satellite optical wireless networks. They are random optical codes (ROCs). This family is specially adapted for optical networks where the number of channels varies, ever large, low bit rate requirements, energy limitations and packed data are present. For example sensor networks have these requirements. The implementation in an intra-spacecraft telecommand and telemeasurement (TC/TM) optical wireless network is also described. Besides we propose a method for the comparison between ROCs and generalized optical orthogonal codes (OOCs). A decision between both family based on the performance for a particular set of parameters is given by the method. An application of the method is also provided. We have compared the families in the intra-spacecraft optical wireless network.

Use of optical orthogonal codes for intra-spacecraft communications

This work presents the application of optical orthogonal codes (OOC) for intra-spacecraft communications within a sensor network with a large number of terminals. In particular, a novel family of optical codes named random optical codes is introduced for the target application. These codes allow a simple optimization of the length and weight of the code while maintaining a controlled probability of error and a low duty cycle. Also, due to a straightforward generation process they can be adjusted almost in real-time to adapt to new conditions like including new channels. This flexibility makes this family a good candidate for systems with strict restrictions over this parameters.

Comparison between the Performance of Algorithmic Optical Codes and Orthogonal Optical Codes in OCDMA Systems

A novel method to be used in OCDMA systems is exhaustively described in this paper. It uses algorithmic optical codes (AOCs). They are based on the signature sequence idea. However patterns are constantly changing. A common seed for pseudo-random sequence is the signature element. In this paper a derivation of the probability of error due to the multiple-access interference is provided. An other contribution of this work is also the comparison between the AOCs and the orthogonal optical codes (OOCs) with (auto- and cross-) correlation equal to one. Attending to the cardinality limitation problem observed in OOCs, the comparison is focused on the number of users allowed by the two different systems aforementioned. As result we state that a system using AOCs allows more users than one using OOCs when the codes are long enough.

Algorithm Optical Codes: An alternative to Random Optical Codes in an intra-satellite optical wireless network

We propose a novel family of optical codes to be used in intra-satellite optical wireless networks. They are algorithm optical codes (ADCs). This family is related with the recently introduced family random optical codes (ROCs). Both families of codes are designed to be used in optical code-division multiple-access (OCDMA) systems. Similarly than their predecessor, i.e. ROCs, ADCs are specially suitable for optical networks where the number of channels varies, ever large, low bit rate requirements, energy limitations and packed data are presented. For example sensor networks demand these requirements. A detailed description of AOCs is shown in this paper. Furthermore a comparison between them and ROCs is presented. We find that AOCs provide less probability of error than ROCs, for a given length of the code and a number of users. Moreover a system using AOCs can change the length of the code easier than a system using ROCs. This fact permits a more efficient accommodation to the actual number of users. Finally the implementation in an intra-spacecraft telecommand and telemeasurement (TC/TM) optical wireless network is also described. We compare the families in the intra-spacecraft optical wireless network.

Models and algorithm for the calculation of the impulse response on IR-wireless indoor channels

Recently there has been growing interest in using infrared (IR) light for broadband indoor wireless communications. There are two major limitations for establishing a wideband infrared communications link. The first and most important limit is the power requirements of such a link. The second important impairment is the intersymbol interference caused by multipath dispersion. The use of angle-diversity receiver allows to achieve high optical gain and a wide field of view simultaneously, it can reduce the impact of ambient light noise, path loss and multipath distortion, in part by exploiting the fact that they are often received from different directions than the desired signal. The advantages achieved depend on how signal received in the different elements are detect and processed. For this reason, we have developed a fast simulation tool that allows to study the influence of the IR channel and to propose new techniques and receiver structures for those systems. The indoor optical channel simulation can significantly benefit the design of high performance IR systems, but requires models that fit correctly the channel characteristics. In contrast to previous works, we define new models for the emitter, lenses, receiver, nonimaging concentrators and reflectors upon which a Monte Carlo ray-tracing algorithm allows to study different links. The inclusion of these models benefit the design of IR links since the are nearer to real behavior than the ideals models. The use of this simulation tool allowed us to analyze the behavior of several links and suggest a configuration of a receiver using angle diversity.

Optical injection-induced timing jitter reduction in gain-switched single-mode vertical-cavity surface-emitting lasers

We report an experimental and theoretical investigation of the effect of optical injection on the characteristics of optical pulses generated by gain-switching a 1550 nm single transverse mode vertical-cavity surface-emitting laser (VCSEL). Under continuous wave operation the VCSEL emits in a linear polarization along the whole current range. The experimental analysis of the effect of external optical injection on the timing jitter, maximum power, and pulse width of optical pulses generated by gain-switching the single mode VCSEL is performed for several repetition rates and for different values of the detuning between the frequency of the optical injection and the VCSEL. Experimental results show that for 1 GHz repetition frequency, jitter reductions greater than 70 % can be obtained over a 47 GHz frequency detuning range with a slight increase of 22% in pulse width with respect to the solitary case. A clear anticorrelation between the maximum power and pulse width is also obtained. A theoretical study is also performed by using a model that incorporates both spatial dependence of carrier density and optical field profiles. The two polarization modes are also taken into account in the model. The theoretical results are in good agreement with the experimental results.

Transmission Performance Improvement of Directly Modulated 1.5 µm VCSEL: Simulations and Experiments

In this work, the transmission performance of a directly modulated 1.5 µm Vertical Cavity Surface Emitting Laser (VCSEL) are investigated with respect to the shape of the driving electrical pulse. A model is developed for the device used and the transmission over standard Single Mode Fiber is simulated for NRZ and RZ formats, at 2.5 Gb/s and 10 Gb/s. Results are reported for propagation lengths between 10 Km and 30 Km. Simulations results are compared with experiments. We found that Q factor and propagation length can be improved if non Gaussian pulses are employed.