This paper describes the activities and results of an ESA-funded project concerned with the assessment of optical amplifier technologies and products for applications in fiber optic subsystems of future satellite payloads.
On-board applications are briefly introduced, together with associated system-level requirements. Optical amplifier technologies, research achievements and products are reviewed. They are compared in terms of current performance, perspectives and suitability for the target space applications. Optical fibre amplifiers, not limited to Erbium-doped amplifiers, Erbium-doped waveguide amplifiers and Semiconductor Optical Amplifiers are covered. The review includes analysis and trade-off of all performance parameters including saturation output power, noise figure, polarisation maintaining capability, wall-plug efficiency, and mass and size.
A selection of optical amplifier products for further evaluation and testing is presented. Results of extensive testing covering both functional performance and environmental behaviour (mechanical, thermal vacuum, radiations) aspects are reported. Most of the work has been completed, but an extension has been proposed for checking and comparing the behaviour of doped fibers under gamma radiation.
Since several years, perspectives and assets offered by photonic technologies compared with their traditional RF counterparts (mass and volume reduction, transparency to RF frequency, RF isolation), make them particularly attractive for space applications [1] and, in particular, telecommunication satellites [2]. However, the development of photonic payload concepts have concurrently risen and made the problem of the ability of optoelectronic components to withstand space environment more and more pressing. Indeed, photonic components used in such photonic payloads architectures come from terrestrial networks applications in order to benefit from research and development in this field.
This paper presents some results obtained in the frame of an ESA-funded project, carried out by Thales Alenia Space France, as prime contractor, and Alter Technology Group Spain (ATG) and Universidad Politecnica de Madrid (UPM), as subcontractors, one objective of which was to assess commercial high frequency optical intensity modulators for space use through a functional and environmental test campaign. Their potential applications in microwave photonic sub-systems of telecom satellite payloads are identified and related requirements are presented. Optical modulator technologies are reviewed and compared through, but not limited to, a specific figure of merit, taking into account two key features of these components : optical insertion loss and RF half-wave voltage. Some conclusions on these different technologies are given, on the basis of the test results, and their suitability for the targeted applications and environment is highlighted.
We present results on the electrical characterization of commercial fiber pigtailed 1. 55 μm 2.5 Gb/s VCSEL based on
InAlGaAs active region, tunnel junction (TJ), air-gap aperture and InAlGaAs/InAlAs mirrors. The current-voltage (I-V)
characteristics were measured and the results were fitted to the analytical expressions of an equivalent circuit considering
the TJ in series with the active junction and a series resistance. Carrier capture/escape effects were considered in order to
account for the reduced value of the drop in the measured differential resistance at threshold. The electrical parameters of
both junctions were determined, showing that the TJ was responsible for most of the voltage drop at threshold. High
frequency electrical impedance measurements were used to determine internal parameters as well as the role of external
parasitics. The results were analyzed using a small signal equivalent circuit which includes the TJ, carrier capture/escape
effects, the cavity parasitics, and the electrical access. A good agreement between the experimental and the equivalent
circuit impedances at different bias was obtained by considering the differential resistances of the active and tunnel
junctions extracted from the I-V characteristics, yielding reasonable values of the dynamic time constants and of the
recombination coefficients.
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.
The mass and the power consumption are the most constrictive restrictions in an Aerostatic Platform. We present in this
work a bidirectional link based on a liquid crystal retro-modulator (LCRM). The use of this device displaces to the
terrestrial station the tasks of powering the laser, and the pointing mechanism. Also, due to the non-conductive
characteristic of the LCRM, the power requirements are low.
The beam aperture of the upward link can be tailored with the compromise between the aperture losses, and the tracking
mechanics. The range of the distances for the system presented is from 2 to 20 km, being the lower limit for the tests
and checking of the system, and the higher the final system for a stratospheric survey Aerostat. The system will be
modified to fulfill the requirements associated to the increasing distances, although the main characteristics (modulation
schemes, modulator, etc.) will be maintained.
The system uses two modulation schemes for each direction: in the uplink a carrier constant envelope FSK modulation
is used. For the down link, part of the optical signal is polarization modulated and reflected to the earth. For increasing
the data rate, a multi-polarization modulation is used. Actually, 32 polarization levels are used, increasing the data rate
in a 5-fold factor. The use of differential scheme, i.e. the State of Polarization (SOP) change defines the data, not the
SOP, avoids the necessity of SOP maintenance in the transmitter.
KEYWORDS: Reflection, Solar concentrators, Receivers, Optical filters, Systems modeling, Monte Carlo methods, Compound parabolic concentrators, Reflectors, Signal detection, Infrared detectors
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.
In this work we present laser-based novel devices that maximize the emitted power for constant eye safety level and beam divergence angle, i.e., without affecting the eye safety classification or the necessary tracking accuracy. This is achieved by breaking the spatial coherence of the beam, which allows the system to be considered as an extended light source. The system comprises a laser, a diffuser, a collimator and, sometimes, other optical elements. As an example, one of the devices is composed of a laser, a Lambertian reflective-type diffuser, and a single-piece reflective-refractive collimator of 20 mm aperture and ultra-high numerical aperture (NA = 1.43), which re-collimates the radiation into 3.5 deg. (full angle). According to the IEC 60825-1:1993 (amendment 2, 2001-01), the Accessible Emission Limit (AEL) (Class 1, wavelength λ = 780 nm, exposure T = 30000 s) for this device is 35.9 times greater than that of a laser with the same divergence angle (15.6 dB), i.e., this device is allowed to emit 35.9 times more power than that of the laser alone with the same divergence angle. The switching time, the beam divergence and the eye safety classification remain the same. This power gain varies with the design conditions. In the cases analyzed it goes from = 8.4 (9.24dB) to 551.3 (27.4 dB).
Early algorithms detecting camera shot transitions were based on the calculation of the difference between a single parameter of two consecutive frames. For many applications, only MPEG video is available; and in this situation, performing previous decompression in order to apply pixel- based algorithms may not be strictly required to achieve reasonable results: an alternative to the pixel-based approaches is to work with frame differences directly extracted from the MPEG compressed sequence. This paper intends to prove that shot detection on this domain can be as precise as that on the uncompressed one. After a discussion on the parameters of the MPEG stream best adapted to calculate an inter-frame difference, we present a distance-independent cut detection method, based on cut modeling and clustering, which obtains results similar to those obtained in the uncompressed domain.
A low complexity system of optical links using indoor unguided infrared channels and direct-sequence code division multiple access is presented. Direct sequence spread spectrum techniques improve performance of optical unguided links with background illumination noise, and multipath propagation. It also allows several users to use code division multiple access, to share the same infrared channel. In the system designed, optical infrared carrier is intensity modulation by a direct-sequence spread spectrum electrical signal, driving the optical emitter. Infrared radiation is directly detected by receiver photodiode.
In this paper, a modified Monte Carlo algorithm for the calculation of the impulse response on infrared wireless indoor channels is presented. This work follows a guideline of studies about the infrared wireless diffuse data communications systems. As is well known, the characteristics of the room where the IR diffuse channel is implemented determine some problems in the communication as can be multipath penalty over the maximum band rate or hidden station situations. Classical algorithms require high computational effort to calculate the impulse response in a regular size room. Monte Carlo offers the possibility of validating the assumptions made for these classic algorithms (basically, the lambertian nature of all reflections) with a computational complexity that is decided by the accuracy desired by the user. It is also an structure that can be easily assumed by a parallel computer architecture. In the other hand, its main drawback is that, for a regular sized room, we need to send much more rays than the components that we receive. This is due to the fact that usually rays are not intercepted by the receiver. We have developed a mixed Monte Carlo-Deterministic algorithm which assures that each ray contributes to the final channel response function each time it rebounds with an obstacle. It increases dramatically the number of contributions and reduces, in the same way, the time required for an accurate simulation. Extensive simulation results are presented. They are compared both with other simulation methods and with measured values. We will demonstrate that the method presented here is much faster than Monte Carlo classical simulation schemes. It can be used like a method of simulation itself or as a validation algorithm for other comparative studies of pulse broadening.
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