The four year FSO link attenuation measurement concurrently with most important meteorological parameters was performed at our mountain observatory Milesovka. In this contribution we summarize and classify different atmospheric phenomena after the FSO link attenuation quantity. For all particular phenomena the CD curves, typical events and simple dependences on relevant atmospheric parameter(s) are presented. We consider the following phenomena (approximate specific attenuation in dB/km in brackets): 1. Fog and cloud (hundreds dB/km) 2. Rain and snow (tens dB/km) 3. Atmospheric turbulence (unit dB) 4. Clear air attenuation due to water vapour (unit dB or less)
In this work we propose a predictive model that allows the study of thermal effects produced when the optical radiation interacts with an esophageal or stomach disease with gold nanoparticles embedded. The model takes into account light distribution in the tumor tissue by means of a Monte Carlo method. Mie theory is used to obtain the gold nanoparticles optical properties and the thermal model employed is based on the bio-heat equation. The complete model was applied to two types of tumoral tissue (squamous cell carcinoma located in the esophagus and adenocarcinoma in the stomach) in order to study the thermal effects induced by the inclusion of gold nanoparticles.
Modern free-space optical (FSO) communication systems in many aspects overcome wire or radio communications. They offer a license-free operation and a large bandwidth. Operation of outdoor FSO links struggles with many atmospheric phenomena that deteriorate phase and amplitude of the transmitted optical beam. Thanks to the recent advancing development, these effects are more or less well understood and described. Goal driven research increased the link availability.<p> </p>Besides increasing the availability of data links it is necessary to focus on the accuracy and reliability of testing optical links. Research of the data optical links is focused on the transmission of a large amount of data whereas the testing FSO link is designed to achieve maximal resolution and sensitivity thus improving accuracy and repeatability of the atmospheric effects measurement. Given the fact that testing links are located in the measured media, they are themselves influenced by it. Phenomena such as the condensation on transceiver windows (rain, frost) and the deviation of the optical beam path caused by the wind are referred to as non-standard effects. Non-standard effects never occur independently; therefore we must always verify the cross-sensitivity of the testing link.<p> </p>In the paper we respond to an increasing number of articles dealing with influence of the atmosphere on the link but ignoring the cross-sensitivity of the testing link on other variables than tested. In conclusion, we carry out qualitative and quantitative analysis of self-identified non-standard effects.
Over the past several decades, free-space optical (FSO) systems have gained a specific place in the wireless technology area. The application of these systems is advantageous for high bandwidths, a license free band and quick installation. The main drawback of FSO systems is their dependence on the state of the atmosphere causing deterioration of the FSO systems availability. One of the atmospheric effects which has an essential impact on the performance of the FSO systems is atmospheric turbulence. Atmospheric turbulence leads to fluctuation of the optical intensity in the plane of the receiving aperture. It has been shown that to reduce the effect of atmospheric turbulence, uniform distribution of the optical intensity within the cross section of the beam in the plane of transmitting aperture (phenomenon of diffraction is neglected) and a suffciently large diameter of the circularly symmetric receiving aperture (to achieve aperture averaging effect) are needed. The main idea of our paper is the problem of beam shaping at the transmitter. In our contribution the technique of transformation of a Gaussian beam into a beam with uniform distribution of optical intensity is discussed. For the mentioned transformation we experimentally tested several shaping methods such as multi aperture beam integrators, diffractive diffusers, etc. Usage of laser sources with different degrees of coherence was considered.<p> </p>The purpose of these techniques is to create an optical beam with uniform distribution of optical intensity on the transmitter output. In order to compare and evaluate the particular shaping techniques, a new Trans- formation Complex Quality (TCQ) parameter was defined. The TCQ parameter indicates the optimal shaping technique and also evaluates the quality of the resulting transformed beam with respect to its resistance towards atmospheric turbulence.
In this paper we propose an analytical approach of describing the tuning properties of the MG-Y laser by means of analytical description of the wavelength dependence of the three tuning currents. The model is based on the theoretical analysis and it was experimentally verified. The results of experiments are included in the second part of the paper. Finally, we conclude with the discussion on the model precision and applications.
Modern wireless optical communication systems in many aspects overcome wire or radio communications. Their
advantages are license-free operation and broad bandwidth that they offer. The medium in free-space optical (FSO) links
is the atmosphere. Operation of outdoor FSO links struggles with many atmospheric phenomena that deteriorate phase
and amplitude of the transmitted optical beam. This beam originates in the transmitter and is affected by its individual
parts, especially by the lens socket and the transmitter aperture, where attenuation and diffraction effects take place.
Both of these phenomena unfavourable influence the beam and cause degradation of link availability, or its total
malfunction. Therefore, both of these phenomena should be modelled and simulated, so that one can judge the link
function prior to the realization of the system. Not only the link availability and reliability are concerned, but also
In addition, the transmitted beam is not, generally speaking, circularly symmetrical, what makes the link simulation
more difficult. In a comprehensive model, it is necessary to take into account the ellipticity of the beam that is restricted
by circularly symmetrical aperture where then the attenuation and diffraction occur. General model is too
computationally extensive; therefore simplification of the calculations by means of analytical and numerical approaches
will be discussed.
Presented model is not only simulated using computer, but also experimentally proven. One can then deduce the ability
of the model to describe the reality and to estimate how far can one go with approximations, i.e. limitations of the model
The importance of free space optical links is increasing. The influence of the atmosphere on the optical beam is one of the most relevant issues to be considered for the quality of the link. In this work we deal with optical wave distribution in a multimode fiber, which can be applied as a primary optical beam source in a transmitter for a free space optical link. The utilization of the fiber instead of a laser diode as the primary source of the optical beam presents certain advantages, the beam spot is circularly symmetrical and can be made optimal by a particular excitation of the fiber, and the head of the link can be based on a completely photonic concept. The optical intensity distribution and the complex degree of coherence at the end of the fiber are modeled. The results show that the optimal beam shape can be obtained in the transmitting fiber.
In an indoor optical wireless link laser beam carrying the information propagates through a room and reflects on walls
and various objects. Multiple reflections and multipath distortions occur when using this link. A power budget of the
indoor optical wireless link and a model of the surface reflectivity are presented in the contribution. The directional
properties of the surface reflectivity are simulated by an empirical mathematical function characterizing reflectivity and
directional reflectance of the surface. In the last part of the paper an experimental setup for verification of the created
empirical function is presented.
The resistance of signal transmission to atmospheric phenomena is possible to solve partially by means of the special
optical intensity distribution at the beam spot, i.e. by means of the special beam profile. In practice the resultant beam
profile can take users of the free space optical link by surprise if only computer models of the optical beams are used
and any diffraction effect at the transmitting lens is not considered. Two models of diffraction of optical beam radiated
from the optical transmitter are presented and two methods of the beam modeling are clarified (the method based on
Bessel function integrating and the method based on FFT). Confirmation of the models elaborated is a part of the
The paper deals with the modeling and measuring of laser beams parameters. Utilization of laser beams is intended for
example in free space optical links with the range about several hundred meters. By the help of submitted laboratory
works the students acquaint with basic parameters and basic characteristics of laser beams, modeling of laser beams, and
with suitable methods used for improvement of transmission reliability.
The laboratory exercise will be aimed on the measurement of energy and geometric properties of laser beam as well as
on the determination of wave properties. We will focus on the mentioned quantities from the sight of total irradiated
optical power, the distribution of optical intensity in laser beam profile and half width of energy equivalent beam. The
spectral characteristics of optical beam and spectral linewidth will be the part of the laboratory exercise.
The contribution has pedagogic character and it is closely adherent to the research realized at our institute. Generally we
are focused on the research of optical wireless links in atmosphere so the solved problem is from the area of current
problems of research.
The paper deals with the results of a propagation study on a fixed hybrid Free Space Optical (FSO) and Radio
Frequency (RF) system operating in 850 nm / 58 GHz bands. Propagation models for the availability assessment of both
FSO and RF links were examined against a comprehensive database of meteorological attenuation events. The
influences of individual hydrometeors were analyzed and the availability performances of the simulated FSO/MMW
hybrid link were evaluated. The study pointed out that visibility and rainfall measurements can be only used for the raw
assessment of availability performance due to the concurrent occurrence of different attenuation effect.
Institute of Atmospheric Physics measures atmospheric attenuation on experimental FSO link on 850 and 1550 nm for
more than one year. Experimental site is located at meteorological observatory on the isolated mountain with frequent
fog, low clouds and strong wind occurrence. Measured attenuation is correlated with the wind turbulence intensity,
visibility and LWC. Simple empirical models estimating attenuation on FSO link from meteorological parameters are
formulated and verified through the experiment described. The paper shows also basic statistical behavior of the longterm
FSO signal level in extreme conditions.
The success of free-space optical links operating indoors or in the atmosphere or deep space shows a good perspective of
the technology. There is no doubt that free-space technology is ready for wide practical application. Optical links have
several advantages in comparison with radio links, namely high transmission rates and high security. Advance in systems
for the detection and tracking of moving objects allowed the development of mobile optical wireless links. This paper is
focused on satellite and mobile optical links. Included are basic characteristics of such links and current results of
international research projects.
Optical communications offer a capable alternative to radio frequency (RF) communications for applications where high
data-rate is required. This technology is particularly promising and challenging in the field of future inter-satellite
communications. The term laser satellite communications (LSC) stands for optical links between satellites and/or high
altitude platforms (HAPs). However, optical links between an earth station and a satellite or HAPs can be also involved.
This work gives an overview of nowadays laser satellite communications. Particularly, it is focused on the factors
causing degradation of the optical beam in the atmosphere. If an optical link passes through the atmosphere, it suffers
from various influences such as attenuation due to absorption and scattering, intensity fluctuations due to atmospheric
turbulence and background radiation. Furthermore, platform vibrations cause mispointing and following tracking losses.
Suitable devices and used pointing and tracking system for laser satellite communications are discussed. At the end,
various scenarios of the optical links and calculations of their power link budgets and limitations are designed.
Implemented software is used for calculation of optical links. This work proves that the Free Space Optics (FSO)
systems on mobile platforms, like satellites and HAPs are a promising solution for future communication networks.
A dual optical wireless test link works with two different optical wavelengths in spectral windows of 850 nm and
1550 nm. The transceivers of the link are placed on the highest peak of the Czech Central Mountains (Milesovka
Mountain) so that the transmission path is oriented almost in a vertical direction. The installation site is situated in a
locality with the harshest climate in the Czech Republic with extreme attenuation conditions. The almost vertically
oriented path of the link allows analyzing the impact of the different atmospheric layers on the signal transmission. The
monitoring of the received power and the archiving of the appropriate data are constantly provided. The relationship
between the link attenuation and the atmospheric visibility has been investigated. The results of this experiment are
The aim of this paper is to design measuring method for determining applicability of picked laser diode in the optical
wireless communications. Chosen laser diode must operate to not debase received optical signal quality and dynamics in
optical wireless link.
We are focusing on the processes in the optical links which are influenced by thermal effects. Varying laser diode's
operating temperature and thermal fluctuations in atmospheric transmission media, in which the information carried by
laser beam is propagating, are considered as the most significant thermal events in optical wireless communications.
As mentioned before, the influence of operating temperature for laser diode's light emission has to be taken into account.
The operation temperature affects on the physical properties of laser diode. Transmitting laser beam is collimated by
transmitting lens but in case of laser diode's operating temperature vary the change in irradiation characteristics can
occurs as an issue in optical wireless links because the distance between laser diode and transmitting lens can't be
adapted by the reason of laser beam divergence settings.
Atmospheric turbulence influence on spectrum and laser beam geometry of laser beam is investigated. In terms of
results' evaluation we can determine when detected signal is acceptable or it doesn't fulfill conditions for signal in
optical wireless communications.
This work deals with a review of the possible principles and construction of the terrestrial optical wireless links and the
problematic of the proper photodiode and laser diode selection according to the wavelength dependent atmospheric
transmittance and the photodiode sensitivity. For the calculations we used the power balance equations and the catalog
values of the available laser diodes and detectors. For the most commonly used free space optical links, the laser diode
price and the maximum power are also considered.
In wireless optical communications, information is carried by coherent laser beams propagating through the free space.
In realized communication channels, the standard beams created directly in the laser resonator are utilized. In recent
time, an increasing attention has been focused on the so-called nondiffracting beams generated by auxiliary optical
systems. In this paper, the theoretical and experimental aspects of the nondiffracting propagation of light are discussed
and geometrical parameters and physical properties of nondiffracting beams promising for wireless communications
We present a compact Extended Cavity Laser (ECL) system based on a high-power laser diode optimized for maximum
efficiency of the Rb optical pumping process. The system represents the crucial part of the HpG (hyperpolarized gasses)
production process. We concentrated on the ECL system optimization - linewidth matching and frequency stabilization - for the optical pumping process. We show that the intensity of optical feedback in the ECL laser influences linewidth and
output power and it is possible to find an optimum value for the highest power spectral density at the absorption line of
desire. The emission linewidth was reduced approximately 10 times with only half of the total optical power loss. The
ECL system is controlled by electronic servo-loop for laser frequency stabilization.
The complex model of terrestrial FSO link is based on two models: the power-budget model of a given link (steady
model) and the installation site model (statistical model). The steady model is represented by the power balance
equation and the power level diagram. The statistical model consists in the knowledge of cumulative exceedance
probability of the random atmospheric attenuation coefficient. The parameters of the statistical model depend only on
the atmospheric phenomena on installation site. Data from more than 200 sites in Italy, France and Germany are
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.
The estimation of FSO link availability is based on synthesis of two models: the steady model of given link and the statistical model of installation site. The steady model consists in the knowledge of FSO transceiver parameters so that we can compute the power budget for a given transceiver distance. The result is the dependence of link margin on the transceiver distance. The statistical model consists in the knowledge of statistical parameters of atmosphere at given installation site. The installation site is described by the exceedance probability of atmospheric attenuation. By means of synthesis of both models we can obtain estimation of link availability. In this contribution, estimation of FSO link availability in chosen Central - European localities is presented.
The paper deals with a statistical model of free-space optical (FSO) data link that takes into account the duration of individual fade events. It is well known that a general data transmission using the Internet protocol (IP) is almost insensitive to link interruptions for hundreds of milliseconds while real-time video services will be badly affected. The statistical model is based on the knowledge of the probability density function of random attenuations and the probability density function of durations of the fades. The paper shows the results from our test site obtained from 1999 to 2001.
This paper presents experimental results obtained by a deposition double-layer system made by means of the electron-beam vacuum evaporation technique. We used short-wavelength 633 -635 nm laser diodes. The wavelengths of these devices are close to the wavelengths of traditional He-Ne lasers. We use them in an extended-cavity laser design for metrological purposes. The resulting reflectivities were evaluated by measuring a testing plate of GaAs and by measuring the "modulation depth" of the coated diode emission spectra. Our best results were obtained for reflectivities well below 10-4 and repeatability ofthe deposition process in a range not exceeding 2x10-4.