KEYWORDS: Free space optics, Turbulence, Wireless communications, Visibility, Data transmission, Prototyping, Visibility through fog, Radio optics, Signal attenuation, Mass attenuation coefficient
The paper presents a construction of a wireless hybrid data link operates applying two transmission channels using optical radiation (FSO – Free Space Optics) and radio one (RF - Radio Frequency). Based on some parameters (e.g. laser power, optics aperture, photodetector detectivity, signal bandwidth, beam divergence) of the link components, its data range was determined for various operating conditions (visibility and turbulence). Preliminary tests of the link prototype (TRL 6) were carried out at the Military Communication Institute, Poland. The results showed that the use of FSO/RF technology systems can provide the increase in data transmission security, link availability, and immunity to intentional interference. Considering the features of this technology, high applicable potential to military activities was observed.
InAsSb ternary alloy is considered to be an alternative to HgCdTe (MCT) in long-wavelength infrared (LWIR) spectral region. The high operation temperature conditions are successfully reached with AIIIBV bariodes, where InAsSb/AlAsSb system is playing dominant role. Since theoretically there is no depletion region in the active layer, the generation-recombination and trap-assisted tunneling mechanisms are suppressed leading to lower dark currents in comparison with standard photodiodes. As a consequence, the bariodes operate at a higher temperature than standard photodiodes which could be used in wide range of system applications, especially where the size, weight, and power consumption are crucial, to include - Free Space Optics (FSO) system. In terms of FSO the LWIR provides limited scattering and atmosphere turbulence in comparison to the commonly used ~ 1550 nm range. Currently, as reported HgCdTe LWIR with peak wavelength ~ 10 μm exhibiting ~ 9×109 cmHz1/2/W (unbiased) and ~ 3×1010 cmHz1/2/W (200 mV) and response time 0.3-4 ns are used. The paper presents detailed analysis of the bariode’s performances (such as dark current, detectivity and response time) versus applied voltage, operating wavelength, temperatures (4-, 2- stage thermoelectrical cooling) and structural parameters (active layer Sb composition, xSb). The optimal working conditions are simulated (non-equilibrium). Theoretical predictions of bariode’s performances are compared with experimental data published for commonly used FSO HgCdTe detectors.
Military wireless communication systems provide services using radio transmission technologies. However, this technology is exposed to modern radio-electronic warfare devices. In some operational scenarios, an application of laser data link (Free Space Optics - FSO) makes it possible to increase data transmission reliability and security. Such capability could be obtain using so-called hybrid FSO/RF communication system. The development of optoelectronic devices (lasers and photodetectors) operated in the spectral range of 8 – 12 μm (Long Wavelength Infrared Radiation - LWIR) enabled to construct a new FSO link. Compared to currently used FSO systems operating in 700-900 nm (NIR) or 1300 – 1550 nm (SWIR) spectral ranges, this link is characterized by less sensitivity to atmospheric phenomena (fog, mists or turbulences) and by greater difficulty of detection. The performed analyses shown that the use of FSO/RF technology systems can provide increase in data transmission security, link availability, secretiveness of operation, and immunity to intentional interference. The paper presents virtues of some wireless communication technologies based on radio, optical and hybrid system configuration. The status of the research on FSO link operated at LWIR wavelength range is also described. This study has been performed in the frame of LasBITer project financed by The Polish National Center of Research and Development. The developed optical data link is a unique device in relation to the FSO technologies available today, because of its operation in LWIR spectrum using quantum cascade lasers and HgCdTe detectors. Scenario of FSO/RF data link works in military operation is also presented.
Noise parameters of a photodetector have a significant role in many science and life activities, e.g. gas spectroscopy, optical communication, objects detection and recognition, construction of laser rangefinders. However, applied noise measurement methods are not so efficient to study new generation of infrared radiation detectors. The paper presents performance comparison of two correlation methods of photodetector signal to determine its noise. These methods apply the signal correlation of a time series with itself (autocorrelation method - ACM) and the similarity between two time series (cross-correlation method with two-channel signal readout - CCM). Both methods were analyzed using a designed simulator (MATLAB/Simulink). It makes it possible to design a special system consisted of electronic block of a twochannel signal readout and signal processing unit. The verification of this setup performances for noise measurement was performed using some low-noise resistors with well-defined resistances. Finally, the setup was also used to measure noise spectral density of MCT detector.
In this paper, the study of AlInAs/InGaAs/InP Quantum Cascade Lasers application in Free Space Optical data link is performed. Implementation of such FSO link operated in long-wavelength infrared (LWIR: 8-12 μm) will be unique for construction of so-called RF/FSO hybrid communication system. The range of longer wavelengths provides better data transfer performance in the case of severe weather conditions, especially, fog, low haze or air turbulence. In the frame of this work, series of QCLs for application in FSO system were examined. They are characterized by different geometries and constructions towards best performance in optical link systems operated in the wavelength range of 8-12 μm. The preliminary test of QCLs included electrical measurements of pulsed light-current-voltage characteristics and time-resolved spectra. The obtained results made it possible to determine operation point for FSO. Their modulation performances were tested using the laboratory laser drivers. Based on measurements, both power and time parameters of QCLs pulses were investigated. These results defined critical values for FSO system. The second part of the analysis concerned the spatial parameters of QCLs radiation. Knowledge of spatial characteristics of emission is vital for FSO optics construction. To characterize spatial properties of beams, far-field patterns of emission were registered. Finally, the obtained results made it possible to optimize the optical transmitter construction and further performance of FSO laboratory model. This research was supported by The Polish National Centre for Research and Development grant DOB-BIO8/01/01/2016.
In this paper some constructions and test results of new detection modules for Free Space Optics (FSO) operated in the 8-12μm wavelength range are presented. In this spectrum, FSO communication is less sensitive to atmosphere features. The main requirements for detectors construction applied in FSO receiver correspond to both operational and functional parameters. These devices should be characterized by high detectivity and operation speed. It is necessary to achieve low value of error rate in the case of high attenuation of laser beam and small-aperture receiving optics. In this way, the design of the FSO transceiver is simplified. In practice, detection performance close to fundamental limits are required. Additionally, for the present optical links, subnanosecond response time is also very important. This can be achieved using of HgCdTe photodiode in the form of modified N+pP+ heterostructure with immersion lens, reverse biasing and TEC cooling. Immersion lens enables optimization of the detector physical dimensions, decreasing detector capacity and time constant detector. The high detectivity of the detection module was achieved by both matching the photodiode to the preamp and minimizing noises. The paper is review of designing and investigation process of the detection modules for FSO application. Some results of the performed simulations and experiments are also discussed. Finally, further refinements will be conducted taking into account parameters of FSO receiver.
There are many room temperature applications to include free space optics (FSO) communication system combining quantum cascade lasers sources where HgCdTe long-wave (8-12 micrometer) infrared radiation (LWIR) detector reaching ultrafast response time < 1 ns and nearly background limited infrared photodetection (BLIP) condition are implemented. Both nearly BLIP detectivity and ultra-response time stay in contradiction in detector’s optimization process. That issue could be circumvented by implementation of the hyperhemispherical GaAs immersion lens into structure to increase optical to electrical area ratio giving flexibility in terms of response time optimization. The optimization approach depends on voltage condition. The generation - recombination (GR) mechanism within active layer was found to be important for forward and weak reverse conditions while photogenerated carrier transport is significant for higher reverse bias. Except of applied voltage, the drift time strongly depends on thickness of the absorption region. Reducing the thickness of the active region, the short drift times could be reached, but that solution significantly reduces quantum efficiency and lowers detectivity. Taking that into consideration a special multilayer heterostructure designs are developed. The p-type absorber is promising due to both high ambipolar mobility and low thermal GR driven by the Auger 7 mechanism. Theoretical simulations indicate that depending on bias condition and T = 300 K the multilayer barrier LWIR HgCdTe structure could reach response time below < 100 ps while biased and ≤ 1 ns while unbiased. Immersed detectivity reaches > 109 cmHz1/2/W. Since commercially available FSO could operate separately in SWIR, MWIR and LWIR range - the dual band detectors should be implemented into FSO. This paper shows theoretical performance of the dual band back-to-back MWIR and LWIR HgCdTe detector operating at 300 K pointing out the MWIR active layer influence on LWIR operating regime.
The paper presents some aspects of approach to construct an optical detection system for laser absorption sensing. These aspects concern a designing procedure of the system, starting with a photodetector, preamplifier, and ending with a signal processing. There is also discussed a project of integrated preamplifier which can be applied in Cavity Enhanced Absorption Spectroscopy (CEAS). In the preamplifier, boxcar method was implemented using integrated circuit (IC) of switched integrator and a special synchronous block. Taking into account an idea of CEAS operation, selected methods of the exponential signal analyzing are described and tested. The main task of the methods is to determine decay time of this signal. The results of these tests make it possible to define some virtues of the designed boxcar technique that could be useful in CEAS sensing instrument.
A laboratory demonstrator of the optoelectronic sensor employing cavity enhanced spectroscopy has been designed to detect the trace amounts of carbon monoxide. High sensitivity of this sensor is provided by the use of optical cavity consisted of dielectric mirrors with extremely high reflectance. The instrument concept was taking into consideration the latest achievement of optoelectronic technology as a part of the ‘EDEN’ project, funded by the Polish National Centre for Research and Development. Preliminary tests using the sensor concept have shown that detection of carbon monoxide is possible using the developed devices. In this configuration, the sensor is characterized by high linear sensitivity in the concentration range of 10 ppb to 2.5 ppm.
The article presents a software ‘SALAS’ (Spectral Analyzer for Laser Absorption Spectroscopy) dedicated to the analysis of gas absorption spectra. The analysis takes into consideration performances of two laser absorption spectroscopy techniques i.e. cavity enhanced absorption spectroscopy (CEAS) and wavelength modulation spectroscopy (WMS)[1]. It is based on data imported from the HITRAN database. The main task is to determine spectral range of operation providing effective results of these gas detection techniques. The SALAS is not only able to identify spectral lines for the defined gas but also to indicate so-called “fingerprint” line with the least impact of other gases (interferents). For this purpose, the software gives the possibility to import data of several gases and to define the detected gas. The implemented procedures indicate the influence of the interferents absorption spectra on the absorbance of the tested gas. This operation is particularly important for CEAS technique in which the main importance is to indicate characteristic absorption line. This provides high sensitivity and measurement selectivity. The developed software was tested for the identification of selected gases, called the disease biomarkers in the exhaled air. These tests were performed in the frame of the project ‘Sensormed’ taking into consideration the detection of nitrous oxide, methane and carbon monoxide[2,3]. Correct operation of the SALAS was also defined by comparing the results of the absorption lines analysis for other gases to the results found in the literature.
The aim of this paper is to address some of the aspects of thermal management of QCLs. Results include electrical and spectral characterization of the devices. Results show shift of QCL emission mode towards lower wavenumbers during the pulse. Characteristics were registered at different temperatures of operation and driving conditions. Registered shift rates depend on operating temperature, being the highest at room temperature. Based on spectral tuning results, temperature increase rates for different modes of operations were evaluated, delivering information on thermal dynamics of investigated devices.
The article describes application of cavity enhanced absorption spectroscopy (CEAS) for detection of nitrogen oxides and vapours of explosives. The oxides are important greenhouse gases that are of large influence on environment, living organisms and human health. These compounds are also markers of some human diseases as well as they are emitted by commonly used explosives. Therefore sensitive nitrogen oxides sensors are of great importance for many applications, e. g. for environment protection (air monitoring), for medicine investigation (analyzing of exhaled air) and finally for explosives detection. In the Institute of Optoelectronics MUT different types of optoelectronic sensors employing CEAS were developed. They were designed to measure trace concentration of nitrogen dioxide, nitric oxide, and nitrous oxide. The sensors provide opportunity for simultaneous measurement of these gases concentration at ppb level. Their sensitivity is comparable with sensitivities of instruments based on other methods, e.g. gas chromatography or mass spectrometry. Our sensors were used for some explosives detection as well. The experiment showed that the sensors provide possibility to detect explosive devices consisting of nitroglycerine, ammonium nitrate, TNT, PETN, RDX and HMX.
The article describes an application of cavity enhanced absorption spectroscopy for nitric oxide and nitrous oxide
detection. Both oxides are important greenhouse gases that are of large influence on environment, living organisms and
human health. These compounds are also biomarkers of some human diseases. They determine the level of acid rain, and
can be used for characterization of specific explosive materials. Therefore the sensitive detectors of these gases are of
great importance for many applications: from routine air monitoring in industrial and intensive traffic areas, to detection
of explosives in airports, finally for medicine investigation, for health care, etc.
Our compact detection system provides opportunity for simultaneous measure of both NO and N2O concentration at ppb
level. Its sensitivity is comparable with sensitivities of instruments based on other methods, e.g. gas chromatography or
mass spectrometry.
The paper presents a model of a free space optical transmitter operating in the wavelength range of 8-12 μm. In the
transmitter, a quantum cascade laser controlled by a special driver was applied. The driver provides the control of the
energy, repetition rate and duration of radiation pulses. In the described transmitter, generation of pulses with high duty
cycle is also provided, making it possible to use the data link with either RZ or NRZ coding. In the frame of the study,
the impact of laser working conditions on the data link operation was determined.
The paper presents a construction and some investigations results of a new driver for quantum cascade (QC) lasers.
The driver is designed to control pulse and CW mode QC lasers. For both laser mode operations, there is the possibility
to set a laser current in the range of 0.1 A - 3 A and pulse duration of 30 ns at the frequencies to the level of 25 MHz.
The stabilization of the temperature in the range of 50°C ÷ -30°C with accuracy of 1.5 K is also achieved. A full
synchronization with external pulses makes the driver very useful in a construction of a free space optical transmission
system and of a laser spectroscopy setup.
The paper presents an application of an optical parametric generator (OPG) for cavity enhanced absorption spectroscopy
(CEAS) technique to nitric oxide (NO) detection. The principle of this method is based on an injection of a radiation
beam into an optical cavity at a very small angle. The radiation is multiple reflected inside the resonator equipped with
spherical and high reflectance mirrors. After each reflection a part of the radiation leaves the optical cavity due to
residual transmission of mirrors. In the case of NO detection the laser emitting at around 5.26 μm was applied. During
investigation of CEAS system with OPG, two-lenses collimator was required to improve OPG beam divergence. The Ge
and ZnSe lenses were used. Thanks to this decrease of about three times in beam diameter was achieved (at the distance
of 1 m from source). It make it possible to measure output signal from the CEAS optical cavity.
The paper presents construction of broadband optical system devoted to free space optical communication using long
wavelength quantum cascade laser and a heterostructural thermally cooled HgCdTe photodetector. This system should
characterize with lower sensitivity to adverse meteorological conditions when compared with the systems operating
in near IR.
The paper presents a procedure and laboratory setup for calibration of extreme ultraviolet detectors. The procedure
determinates a responsivity of a photodiode applied in the technology of EUV radiation. The main elements of the
described system are a gas-puff laser plasma source, metrology chamber with an optical beam splitter and a model
instrument. The applied measurement method is based on a comparison of the detected signals from the model
instrument and the tested detector. The reduction of an error resulting from instabilities of the source radiation is
ensured using a special construction of the optical beam splitter. Because of the cost and the metrology features,
the presented setup is future alternative for small factories and laboratories producing EUV detectors. In the paper,
analysis and preliminary research of the setup are described.
The paper presents a measuring system of extreme ultraviolet radiation pulses (13.5 nm). The system is used for
monitoring a gas-puff laser-plasma source constructed at the Institute of Optoelectronics. The radiation source and the
system are used in metrology of EUV optics. The system consists of a detection head and a system of optical filters,
which are housing in a special construction. Additional element of the measuring system is a special processing unit.
The measuring system was used during investigations of the plasma-laser optimization. The results were comparable
with the ones from a spectrograph and an Emon energy meter.
The results of the spectral and spatial measurements of a laser-produced (LPP) plasma source of extreme ultraviolet (EUV) for 13.5 nm are presented. The double stream Xe/He gas puff target in the source was utilizing. The source was equipped with a Nd: YAG laser system (E = 0.55 J, t = 3.9 ns, f = 10 Hz, M2 = 2.5) and was dedicated to EUV metrology purposes. An advantage of this approach of the laser-plasma source is a debris free EUV emission. For the spectral research a transmission grating spectrograph (TGS) and a grazing incidence spectrograph (GIS) were utilized. For spatial measurements a pinhole camera was employed. The influence on EUV emission of the laser focal spot position in relation to the gas puff target and the time delays between opening valves and the laser pulse were investigated.
The paper presents analyses of a testing system of extreme ultraviolet detectors. The testing procedure concerns determination of a quantum efficiency of photodiode detectors. The testing method is based on a comparison of the detected signals from the model detector and the tested one The system consists of a gas-puff laser plasma source, a metrology chamber with an optical system, and a model energy detector. Theoretical and experimental investigations, including optimisation of efficiency and stability of the radiation source, calculation of a charge measurement accuracy of the model detector, determination of mirrors reflectivity and its angle dependence are discussed.
We present an experiment on detection of nitrogen dioxide in free air using cavity enhanced spectroscopy. As a light source a blue pulsed diode laser was applied, while the output signal was detected with a photomultiplier. The absorber concentration was found by investigation of the optical resonator quality. It was done by determination of decay time of radiation pulse trapped in the cavity. Also the measurement of the phase shift between the output signal and modulation signal was used as the alternative method. The detection limit better than 1 ppb was demonstrated. The aim of this experiment was to study potential application of cavity enhanced absorption spectroscopy for construction of fully optoelectronic NO2 detector which could replace the commonly used chemical detectors.
Single ≤1 kJ pulses from a high-power laser are focused into molecular gases to create large laser sparks. This provides a unique way to mimic the chemical effects of high-energy-density events in planetary atmospheres (cometary impact, lightning) matching the natural energy-density, its spatio-temporal evolution and plasma-volume scaling of such events in a fully-controlled laboratory environment. Some chemical reactions initiated by laser-induced dielectric breakdown (LIDB) in both pure molecular gases and mixtures related to the chemical evolution of the Earth's early atmosphere were studied. Most of the experiments were carried out in a static gas cell. However, an initial series of experiments was also performed with a gas-puff target placed within a vacuum interaction chamber. Under these dynamic conditions the hot core of a laser spark can be directly investigated.
For conventional wavelength (UV-vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (<100 nm) lasers emitting pulses with durations ranging from ~10 fs to ~1 ns have recently been put into routine operation. This makes it possible to investigate how ablation characteristics depend on pulse duration in the XUV spectral region. Four sources of intense short-wavelength radiation available in the authors' laboratories, including XUV and soft x-ray lasers, are used for the ablation experiments. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for nanosecond pulses with those measured for shorter pulses, we can study the influence of pulse duration on XUV ablation efficiency. The results of the experiments also show that the ablation rate increases while the wavelength decreases from the XUV spectral region toward x-rays, mainly due to increase of attenuation lengths at short wavelengths.
Experiments on direct photo-etching of organic polymers induced by high-intensity nanosecond pulses of soft X-ray radiation from a laser plasma X-ray source based on a gas puff target are presented. X-rays in the wavelength range from about 1 nm to 8 nm were produced by irradiation of the xenon/helium double-stream gas puff target with laser pulses from the Prague Asterix Laser System (PALS). The resulting X-ray pulses were used to irradiate samples from organic polymers and form microstructures. The results show relatively high efficiency of X-ray direct photo-etching that could be useful for micromachining of organic polymers.
For conventional wavelength (UV-Vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (l < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes it possible to investigate how the ablation characteristics depend on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser (Colorado State University, Fort Collins), focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with those measured for ultrashort pulses, we can study the influence of pulse duration on XUV ablation efficiency. Ablation efficiencies measured with short pulses at various wavelengths (i.e. 86/46.9/21.2 nm from the above-mentioned lasers and ~ 1 nm from the double stream gas-puff Xe plasma source driven by PALS) show that the wavelength influences the etch rate mainly through the different attenuation lengths.
In the paper a newly developed compact laser plasma EUV source is presented. The source is based on the double-stream gas puff target approach. The targets are formed by pulsed injection of high-Z gas (xenon) into a hollow stream of low-Z gas (helium) using the valve system composed of two electromagnetic valves and equipped with the double-nozzle setup. The outer stream of gas confines the inner stream improving the gas puff target characteristics (higher density of high-Z gas at longer distance from the nozzle output). It causes efficient absorption of laser energy in a plasma and strong EUV production. The source has been developed in the frame of the EUV sources development project under the MEDEA+ program.
Spectral measurements of a laser-produced soft x-ray and EUV source based on a double-stream gas puff target are described. The target was irradiated with a Nd:glass laser producing 1 ns pulses with energy up to 10 J. Production in the wavelength range up to 20 nm (x-ray and EUV emissions) have been measured from xenon, krypton, argon, and nitrogen targets using the transmission grating spectrometer with the back-illuminated CCD. Spectral characteristics of x-ray and EUV emissions are presented.
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