Low-frequency noise measurements of IR photodetectors with voltage cross correlation system

Abstract The paper presents a system for noise measurements in infrared photodetectors characterized by low shunt resistances based on a two-channel ultra-low-noise voltage amplifier with paralleled discrete JFETs at the input stages. Using cross correlation method, a background noise well below of 10−19 V2/Hz can be obtained at frequencies above 10 Hz. To facilitate the estimation of the noise in such a wide frequency range (5 decades), we also developed a software based on the QLSA library. As a result of these efforts, the equivalent input voltage noise of the system is below 10−19 V2/Hz at 10 Hz and 10−20 V2/Hz for frequencies above a few hundred Hz. The system effectiveness is demonstrated by noise measurements at room temperature on advanced InAsxSb1-x photodetectors characterized by an active area of 1 mm2 and a shunt resistance below 10 Ω

Optical wireless communications operated at long-wave infrared radiation

This paper presents some construction analysis and test results of a Free Space Optics system operating at the wavelength of 9.35 µm. In the system, a quantum cascade laser and a photoreceiver with mercury cadmium telluride photodetectors were used. The main parameters of these elements were discussed taking into account a data link operation. It also provides to determine a data range for various weather conditions related to scattering and scintillation. The results of numerical analyses defined the properties of currently available FSO technologies working in the near infrared or in the short infrared range of spectrum versus the performances of the developed system. The operation of this system was verified in three different test environments. The obtained results may also contain important issues related to the practical application of any FSO system

Low-Noise Programmable Voltage Source

This paper presents the design and testing of a low-noise programmable voltage source. Such a piece of instrumentation is often required as part of the measurement setup needed to test electronic devices without introducing noise from the power supply (such as photodetectors, resistors or transistors). Although its construction is based on known configurations, here the discussion is focused on the characterization and the minimization of the output noise, especially at very low frequencies. The design relies on a digital-to-analog converter, proper lowpass filters, and a low-noise Junction Field-Effect Transistors (JFET) based voltage follower. Because of the very low level of output noise, in some cases we had to resort to cross-correlation in order to reduce the background noise of the amplifiers used for the characterization of the programmable source. Indeed, when two paralleled IF9030 JFETs are used in the voltage follower, the output noise can be as low as 3 nV/√Hz, 0.6 nV/√Hz and 0.4 nV/√Hz at 1 Hz, 10 Hz and 100 Hz, respectively. The output voltage drift was also characterized and a stability of ±25 µV over 3 h was obtained. In order to better appreciate the performance of the low-noise voltage source that we have designed, its noise performances were compared with those of a set-up based on one of the best low-noise solid-state voltage regulators available on the market. Actual measurements of the current noise in a type-II superlattice photodetector are reported in which the programmable source was used to provide the voltage bias to the device

Two-Channel Detecting Sensor with Signal Cross-Correlation for FTIR Instruments

This paper’s purpose was to demonstrate a performance of a novel approach in a low-noise optical sensor for an FTIR spectrometer. Methods: Compared to the standard FTIR detection setup, our sensor ensures a higher signal-to-noise ratio (SNR) and lower signal standard deviation by reducing the uncorrelated noise components (e.g., thermal and 1/f noises of the detection module). Its construction is based on two-channel detection modules and a processing unit with implemented cross-correlation signal analyses. Each module was built of LWIR HgCdTe photodiodes and low-noise transimpedance amplifiers. Results: the experiments demonstrated a decrease in a signal standard deviation of about 1.7 times with a 10 dB-improvement in the SNR. Conclusion: this result indicates our sensor’s main benefit, especially in registered “weak” and noisy interferograms

Radar target imitator

W referacie opisano zaprojektowany i wykonany praktycznie jednokanałowy imitator sygnałów radiolokacyjnych, który pozwala na generację sygnału odbitego od celu o zadanej trajektorii, dla pasma częstotliwości pośredniej. Przedstawione w referacie stanowisko może być wykorzystane do weryfikacji analogowego traktu przetwarzania sygnału radiolokacyjnego na częstotliwości pośredniej oraz algorytmów DSP wykorzystywanych do estymacji współrzędnych imitowanego celu.The paper describes a single channel imitator for radar signals which was designed and made practically to generate a signal reflected from an aerial target with a given trajectory on the intermediate frequency band. Presented setup can be used to verify the analog channel for radar signal processing at intermediate frequencies and the DSP algorithms used to estimate coordinates of the imitated target

Indicator Values of Emergent Vegetation in Overgrowing Lakes in Relation to Water and Sediment Chemistry

Lake overgrowth is one of the mechanisms affecting the gradual disappearance of lakes in the temperate zone caused by excessive eutrophication of waters. The aim of this study was to assess the possibility of using helophytes as bioindicators of lake overgrowth based on long-term changes based on the cartographic maps studies (1911–2012) and field analyses of plant form growth cover (2012–2014). Additionally, water and sediment chemistry in overgrowing lakes were investigated. The study comprised a total of 33 eutrophic lakes characterized by varying degrees of vegetation development. Based on discriminant analysis, four groups of lakes with diverse degrees of vegetation growth were distinguished. The group comprising the most overgrown water bodies was found to have the greatest percentage of helophytes, with a considerable proportion of submerged and floating-leaved macrophytes. Based on a review of archival materials, lakes which currently exhibit the highest degree of overgrowth were shown to have been affected by intensive littoral growth for over 100 years, which confirms bioindicator properties of helophytes in the assessment of the degree of lake overgrowth. In addition, lakes with the highest rate of overgrowth were characterized by a high content of nitrogen in sediment, with a concurrent high concentration of potassium

Application of cross-correlation-based transimpedance amplifier in InAs and InAsSb IR detectors noise measurements

The paper presents noise measurements in low-resistance photodetectors using a crosscorrelation-based transimpedance amplifier. Such measurements usually apply a transimpedance amplifier design to provide a current fluctuation amplification. In the case of low-resistance sources, the measurement system causes additional relevant system noise which can be higher than noise generated in a tested detector. It mainly comes from the equivalent input voltage noise of the transimpedance amplifier. In this work, the unique circuit and a three-step procedure were used to reduce the floor noise, covering the measured infrared detector noise, mainly when operating with no-bias or low-bias voltage. The modified circuit and procedure to measure the noise of unbiased and biased detectors characterized by resistances much lower than 100 Ω were presented. Under low biases, the reference low-resistance resistors tested the measurement system operation and techniques. After the system verification, noise characteristics in low-resistance InAs and InAsSb infrared detectors were also measured

FET input voltage amplifier for low frequency noise measurements

The paper presents a low noise voltage FET amplifier for low frequency noise measurements. It was built using two stages of an op amp trans impedance amplifier. To reduce voltage noise, eight-paralleled low noise discrete JFETs were used in the first stage. The designed amplifier was then compared to commercial ones. Its measured value of voltage noise spectral density is around 24 nV/√Hz, 3 nV/√Hz, 0.95 nV/√Hz and 0.6 nV/√Hz at the frequency of 0.1, 1, 10 and 100 Hz, respectively. A -3dB frequency response is from ~20 mHz to ~600 kHz

Cross-correlation method for noise measurements of photodetectors used for laser absorption spectroscopy

W pracy przedstawiono wyniki pomiarów gęstości widmowej napięcia szumów detektorów fotonowych o małych rezystancjach przy użyciu specjalnie opracowanego stanowiska pomiarowego. Badania tych detektorów mają duże znaczenie dla wielu aplikacji. Są one szczególnie istotne dla układów laserowej spektroskopii absorpcyjnej do wykrywania śladowych ilości gazów. Uzyskiwana w nich granica wykrywalności jest bezpośrednio związana nie tylko z szumami źródeł promieniowania i szumem tła, lecz także z szumami detektora oraz kolejnych stopni fotoodbiornika. Zastosowanie w opracowanym systemie specjalnie zaprojektowanych ultramałoszumowych torów pomiarowych (wzmacniacze o napięciu szumów 3,6 × 10⁻¹⁹ V² /Hz dla f > 1 kHz) oraz operacji korelacji sygnałów w czasie 10 minut umożliwiło uzyskanie szumu tła poniżej 10⁻¹⁸ V² /Hz dla f > 10 Hz oraz poniżej 10⁻¹⁹ V² /Hz dla f > 1 kHz. Efektywność systemu zweryfikowano poprzez pomiary referencyjnych rezystorów, a następnie detektora z supersieci drugiego rodzaju (T2SL) wykonanego z InAs/InAsSb.The paper presents noise measurements of low-resistance photon detectors with a specially developed system. These measurements are significant for many applications. This issue is particularly critical for laser absorption spectroscopy systems to detect trace amounts of gases. In these systems, the detection limit is determined by noise origins, e.g., light source, background, and detector noise and its readout electronics. The use of some specially designed components of the system (low-noise - 3.6 × 10⁻¹⁹ V² /Hz for f >1 kHz) cross-correlation signal processing provides to obtain a measuring floor noise below 10⁻¹⁸ V² /Hz for f > 10 Hz and below 10⁻¹⁹ V² /Hz for f > 1 kHz after ten minutes’ analysis. Measurements of some reference resistors have verified the system’s performance. Finally, the system was also applied to determine the spectral noise density of the II-Type SuperLattice photodetector made of InAs/InAsSb

Long-Term Changes in and Conservation Guidelines for Water Caltrop (<i>Trapa natans</i> L.) in Two Reservoirs in Poland

Climate change, worsening freshwater quality, and anthropogenic factors have caused water caltrop to lose approximately 80% of its habitat in Poland since the early 1980s. The presence of this plant species has substantially changed since the end of the 19th century. Our aim in this study was to examine the habitat and conservation status of Trapa natans in the Szumirad reservoir and Nowokuźnicki pond reserve in Poland and to indicate sources of potential hazards for the analyzed population. To achieve this aim, we spatially analyzed the changes in the total reservoir area, dynamics of species population, physico-chemical parameters of water, and climatic data. For the Szumirad reservoir, we observed substantial changes in water caltrop quantity and condition. For the Nowokuźnicki pond reserve, we found a serious threat to the Trapa habitat posed by the developing Nupharo–Nymphaeetum albae association, which is a strong competitor of nymphaeids. The obtained results indicated that surface waters localized in protected areas might play an important role in maintaining the population of water caltrop. On the basis of our analyses of selected populations, we emphasize that present protection procedures should be supplemented with the active protection of the species