Integration Time Of Measurement Research Articles

Demonstration of the First Outdoor 2-μm-Band Real-Time Video Transmission Free-Space Optical Communication System Using a Self-Designed Single-Frequency Fiber Laser

In the present paper, we demonstrate the first outdoor free-space optical communication (FSOC) system with real-time video transmission in a 2-μm-band with a state-of-the-art data rate performance of 10 Gbit/s and a communication distance of ∼177 m. The measured wide-open eye diagram and zero bit-error-rate also validated the high communication quality. The FSOC system is constructed with all homemade components, including a single-frequency (SF) thulium-holmium co-doped fiber laser (THDFL), a modulation module, a demodulation module, three thulium-doped fiber amplifiers, an optical bandpass filter, two optical-antennas, and a display system. As the most important component, the SF THDFL, enabled by a uniform fiber Bragg grating and a passive fiber ring-compound cavity, is self-designed and fabricated by our group, presenting high lasing stability centered at 2048.98 nm, an optical signal-to-noise ratio of >81 dB, a relative intensity noise of <-128.18 dB/Hz at >0.5 MHz and a linewidth of 5.290 kHz under a measurement integration time of 0.001 s. Our work is an important step forward for practical outdoor FSOC in a 2-μm-band, proving useful for engineers and scientists when our SF THDFL is implemented as a low-cost, high-performance laser source for the FSOC system.

Single Transmitter Direction Finding Using a Single Moving Omnidirectional Antenna

Traditional direction-finding systems are based on processing the outputs of multiple spatially separated antennas. The impinging signal Angle-of-Arrival (AOA) is estimated using the relative phase and amplitude of the multiple outputs that are sampled simultaneously. Here, we explore the potential of a single moving antenna to provide useful direction finding of a single transmitter. If the transmitted signal frequency is steady enough during the collection of data, a single antenna can be moved while tracking the phase changes to provide an Angle-of-Arrival measurement. The advantages of a single-antenna sensor include the sensor size, the lack of a need for multiple-receiver synchronization in time and frequency, the lack of mutual antenna coupling, and the cost of the system. However, a single-antenna sensor requires an accurate knowledge of its position during the data collection and it is challenged by transmitter phase instability, signal modulation, and transmitter movement during the measurement integration time. We analyze the performance of the proposed sensor, support the analysis with simulations and finally, present measurements performed by hardware configured to check the validity of the proposed single-antenna sensor.

Detection statistics for coherent RMCW LiDAR.

This paper presents an analytical model and experimental validation for the detection performance and false-alarm rates for phase-encoded random modulation continuous-wave (RMCW) LiDAR. Derivation of the model focuses on propagating the effects of relevant noise sources through the system to determine an analytical expression for the detection rate, expressed by the probability of detection. The model demonstrates that probability of detection depends only on three factors: i) the mean signal-to-noise ratio (SNR) of the measurement; ii) the measurement integration time; and iii) speckle-induced intensity noise. The predicted analytical relationship between measurement SNR and probability of detection was validated by numerical simulations and experimental demonstrations in both a controlled fiber channel and under fully-developed speckle conditions in an uncontrolled free-space channel.

Gate voltage dependence of noise distribution in radio-frequency reflectometry in gallium arsenide quantum dots

We investigate gate voltage dependence of electrical readout noise in high-speed rf reflectometry using gallium arsenide quantum dots. The fast Fourier transform spectrum from the real time measurement reflects build-in device noise and circuit noise including the resonator and the amplifier. We separate their noise spectral components by model analysis. Detail of gate voltage dependence of the flicker noise is investigated and compared to the charge sensor sensitivity. We point out that the dominant component of the readout noise changes by the measurement integration time.

Beamline characterization of a dielectric-filled reentrant cavity resonator as beam current monitor for a medical cyclotron facility.

At PSI (Paul Scherrer Institute), Switzerland, a superconducting cyclotron called "COMET" delivers proton beam of 250MeV pulsed at 72.85MHz for proton radiation therapy. Measuring proton beam currents (0.1-10nA) is of crucial importance for the treatment safety and is usually performed with invasive monitors such as ionisation chambers (ICs) which degrade the beam quality. A new non-invasive beam current monitor working on the principle of electromagnetic resonance is built to replace ICs in order to preserve the beam quality delivered. The fundamental resonance frequency of the resonator is tuned to 145.7MHz, which is the second harmonic of the pulse rate, so it provides signals proportional to beam current. The cavity resonator installed in the beamline of the COMET is designed to measure beam currents for the energy range 238-70MeV. Good agreement is reached between expected and measured resonator response over the energy range of interest. The resonator can deliver beam current information down to 0.15nA for a measurement integration time of 1s. The cavity resonator might be applied serving as a safety monitor to trigger interlocks within the existing domain of proton radiation therapy. Low beam currents limit the abilities to detect sufficiently, however, with the potential implementation of FLASH proton therapy, the application of cavity resonator as an online beam-monitoring device is feasible.

Signal-to-noise ratio limitations for intensity correlation imaging.

Intensity correlation imaging (ICI) is a concept which has been considered for the task of providing images of satellites in geosynchronous orbit using ground-based equipment. This concept is based on the intensity interferometer principle first developed by Hanbury Brown and Twiss. It is the objective of this paper to establish that a sun-lit geosynchronous satellite is too faint a target object to allow intensity interferometry to be used in developing image information about it-at least not in a reasonable time and with a reasonable amount of equipment. An analytic treatment of the basic phenomena is presented. This is an analysis of one aspect of the statistics of the very high frequency random variations of a very narrow portion of the optical spectra of the incoherent (black-body like-actually reflected sunlight) radiation from the satellite, an analysis showing that the covariance of this radiation as measured by a pair of ground-based telescopes is directly proportional to the square of the magnitude of one component of the Fourier transform of the image of the satellite-the component being the one for a spatial frequency whose value is determined by the separation of the two telescopes. This analysis establishes the magnitude of the covariance. A second portion of the analysis considers shot-noise effects. It is shown that even with much less than one photodetection event (pde) per signal integration time an unbiased estimate of the covariance of the optical field's random variations can be developed. Also, a result is developed for the standard deviation to be associated with the estimated value of the covariance. From these results an expression is developed for what may be called the signal-to-noise ratio to be associated with an estimate of the covariance. This signal-to-noise ratio, it turns out, does not depend on the measurement's integration time, Δt (in seconds), or on the optical spectral bandwidth, Δν (in Hertz), utilized-so long as ΔtΔν≫1, which condition it would be hard to violate. It is estimated that for a D=3.16 m diameter satellite, with a pair of D=1.0 m diameter telescopes (which value of D probably represents an upper limit on allowable aperture diameter since the telescope aperture must be much too small to even resolve the size of the satellite) at least N=2.55×10(16) separate pairs of (one integration time, pde count) measurement values must be collected to achieve just a 10 dB signal-to-noise ratio. Working with 10 pairs of telescopes (all with the same separation), and with 10 nearly adjacent and each very narrow spectral bands extracted from the light collected by each of the telescope-so that for each measurement integration time there would be 100 pairs of measurement values available-and with an integration time as short as Δt=1 ns, it would take T=2.55×10(5) s or about 71 h to collect the data for just a single spatial frequency component of the image of the satellite. It is on this basis that it is concluded that the ICI concept does not seem likely to be able to provide a timely responsive capability for the imaging of geosynchronous satellites.

Modeling and measurements of angular truncation for an aerosol albedometer

In this work, we examine the angular truncation behavior and present correction factors for the aerosol albedometer previously developed in our laboratory. This new instrument makes simultaneous measurement of extinction and scattering coefficients (bext and bscat) on dispersed aerosol samples. The aerosol extinction coefficient is measured with cavity ring-down spectroscopy (CRDS), and the scattering coefficient is determined through the integrating sphere nephelometer. However, all nephelometers are not able to collect light scattered from an aerosol sample very near the forward (0°) and reverse (180°) directions, due to the geometrical constraints. This can result in systematic underestimation of scattering coefficient known as truncation error. In order to account for this problem and describe scattering by aerosols more precisely, correction factors (C) for this angular non-ideality have been theoretically developed. Truncation angles (θ) were calculated upon consideration of the geometry of the sphere nephelometer. As truncation error largely depends on particle size and refractive index, C values were computed for a series of spherical, homogeneous aerosol particles with different known particle sizes and refractive indices by Lorenz-Mie theory. Measurements on size-selected, laboratory generated aerosols of known size and composition allowed empirical measurement of truncation correction factors to compare with the Mie model results. Results indicate the model we built overestimates the fraction of light not collected by the sphere. Empirically observed correction factors of ≤ 1.12 for particles with size parameters (α) &lt; 6 were determined. In addition, the effect of number of particles within the probe beam on the suitability of correction factors was also examined. Observations support the hypothesis that particles are rapidly transported / mixed through the probe beam, and measurement integration times as short as 52 s yield data that is indistinguishable from the probe region being homogeneously filled with aerosol, even at very low particle concentrations.

High resolution on-chip thermometry using a microstrip-coupled transition edge sensor

Our recent work demonstrated highly efficient coupling of broadband thermal photon radiation between the termination resistors of a superconducting microstrip transmission line measured using a transition edge sensor (TES). A simple modification of this scheme is presented that permits rapid thermometry of micron-scale objects at temperatures below 3 K. Broadband photon noise gives a limiting temperature sensitivity of 3.8 μK for a 1 s integration time for measurements at 0.5 K. In practice, phonon noise in the thermal link between the TES and the heat bath limits the achievable temperature resolution to about 30 μK for a typical TES with noise equivalent power of 2×10−17 W/Hz with the same integration time.

Use of CO2 Concentration Difference or CO2 Balance to Assess Ventilation Rate of Broiler Houses

Ventilation rate (VR) is one of the two key elements for quantifying aerial emissions from animal production facilities. Direct, continuous measurement of building VR can be challenging and impractical under certain circumstances, e.g., naturally ventilated animal housing or a large number of ventilation fans in the building. This study examined the suitability of estimating VR of broiler houses with built-up litter (mixture of manure and bedding), when supplemental heating was not in use, through either carbon dioxide (CO 2 ) balance or the relationship of VR to CO 2 concentration difference between exhaust and inlet air. The reference VR was based on direct measurement by continuously monitoring operation of the in-situ calibrated exhaust fans. The comparative analysis of the direct method vs. each indirect method was conducted for a measurement integration time (MIT) of 10, 30, 60, or 120 min. The analyses revealed that MIT of 30 min or greater resulted in non-significant differences in VR between the indirect and direct methods. The broiler building VR (m 3 s -1 ) may be related to the exhaust-inlet CO 2 concentration difference (ΔCO 2 , ppm) as VR (±3.0) = 4456 (±41) ΔCO 2 -0.786 (±0.019) at 30 min MIT. The VR may also be determined by the CO 2 balance method (including litter CO 2 generation) with a correction factor of 0.97 at MIT of 30 to 120 min. If litter CO 2 generation is omitted from the total building CO 2 production, the actual VR may be estimated by applying a correction factor of 1.077 to the bird respiration CO 2 balance VR. Hence, the CO 2 balance or concentration difference method offers a viable alternative or supplemental check for quantifying building VR under certain conditions where direct, continuous VR measurement is not feasible.

Polarization-insensitive ultralow-power second-harmonic generation frequency-resolved optical gating

We demonstrate polarization-insensitive ultralow-power second-harmonic generation frequency-resolved optical gating (FROG) measurements with a fiber-pigtailed, aperiodically poled lithium niobate waveguide. By scrambling the polarization much faster than the measurement integration time, we eliminate the impairment that frequency-independent random polarization fluctuations induce in FROG measurements. As a result we are able to retrieve intensity and phase profiles of few hundred femtosecond optical pulses with 50 MHz repetition rates at 5.2 nW coupled average power without control of the input polarization.

Detection of unknown localized contamination on silicon wafer surface by sweeping-total reflection X-ray fluorescence analysis

A new interpretation of the data from Sweeping-TXRF (total reflection X-ray fluorescence) was proposed for the analysis of semiconductor contamination. Formerly, we focused on the accumulated spectrum that represents average concentration. In the proposed method, the individual spectra are also utilized to obtain rough mapping information for the entire wafer surface. Although the individual integration time of measurement is very short (4–8 s/spot), a limit of detection at the level of 10 10 atoms cm −2 is maintained for each spot. This method was used to analyze actual wafers that had particulate contaminants on them, and the capability of particle detection was demonstrated. In addition, this method simultaneously gives the average concentration by using the accumulated spectrum, as reported before. Dedicated software for Sweeping-TXRF is under development and has already achieved a throughput of ca. 30 min for 200-mm wafers and ca. 50 min for 300-mm wafers.

Comparison of self-referencing techniques for photothermal detection of trace compounds in water

Self-referencing techniques are compared for a closed-cell photothermal detector that uses a water meniscus as a pressure sensor. Deflection of the meniscus was measured using an optical fibre Fabry–Perot interferometer. For long measurement integration times, interference fringe drift was a serious limitation on the detection repeatability for non self-referenced measurements. Two self-referencing techniques were compared for measurements of optical absorption. The first technique used a simultaneous reference absorption signal at a second wavelength, and the second used a simultaneous volumetric modulation within the cell. Both methods have been evaluated with photothermal excitation by a 658-nm LED, a 478-nm LED and a UV discharge lamp. For the detection of absorption in aqueous solutions, the two methods had similar performance. However, the volumetric method could be used for detection of any absorbing compound, regardless of its absorption spectrum and was more convenient to use.

Appropriate bandwidth and integration time for measurements of the degree of interaural cross correlation as a measure of apparent source width in concert halls

ISO 3382 Annex suggests using the degree of interaural cross correlation (ICC) as a measure of apparent source width (ASW), while mentioning that the choice of bandwidth for test signals and the integration time are uncertain. Needless to say, what is important for a physical measure is that it is well correlated with the subjective effect, in this case ASW. This paper describes psychological experiments to investigate the appropriate bandwidth for test signals and the integration time for measuring ICC to evaluate ASW. The experimental results indicate that (1) measurements of ICC with 1/3-oct bands are preferred for evaluating ASW, whereas the use of wide band and 1/1-oct band signals as described in the ISO standard are not, and (2) measurement of ICC for all reflections including both the early reflections and later reverberant sound is most suitable for evaluating ASW, rather than the early reflections alone.

Zero dead-zone OTDR with high-spatial resolution for short haul applications

The existence of dead-zones is an important drawback for optical time-domain reflectometers (OTDR), specially in short haul applications with a large number of optical components. In this letter, we present an OTDR system with high spatial resolution based on the time-correlated single photon counting (TC-SPC) technique. This system overcomes the deadzone problem by monitoring the spontaneous Stokes Raman emission (SSRE) to avoid the intense Fresnel reflections. The high sensitivity of the TC-SPC technique (-120 dBm) permits a high-loss budget within reasonable measurement integration times.

Measurement of nitric oxide with an antimonide diode laser

An antimonide diode laser operating near 2.65 mum was used to measure absorption lines of NO gas in the first overtone band. A blended line pair of NO that is sufficiently free of interference from H(2) O to permit the selective detection of NO under reduced pressure conditions was identified. With wavelength-modulation spectroscopy, a rms noise level equivalent to an absorbance of 3.2 x 10(-5) was achieved at a measurement integration time (for a single spectral data point) of 0.1 s. The corresponding detection sensitivity (signal-to-noise ratio of 2) for NO in air at reduced pressure was ~15 ppm m (ppm is parts in 10(6)). Antimonide diode lasers show substantial promise for gas-sensing applications because they can gain access to relatively strong absorption lines of several gases of environmental interest at operating wavelengths at which cryogenic cooling is not required.

Application of orthogonal codes to the calibration of active phased array antennas for communication satellites

This work describes two algorithms designed for remote calibration of an N/sub c/-element active phased-array antenna. These algorithms involve transmission of N/spl ges/N/sub c/ time multiplexed orthogonal encoded signals. The received signals are coherently detected, accumulated in vector forms, and decoded with the inverse of the orthogonal encoding matrix. The unitary transform encoding (UTE) algorithm is most suited for digital beamforming as it requires additional encoding hardware for an analog implementation. The control circuit encoding (CCE) algorithm is ideally suited for analog beamformers as it requires no additional encoding hardware. The CCE method encodes phased-array elemental signals using a Hadamard matrix to control the switching of intrinsic phase shifter delay circuits. The UTE and CCE algorithms can reduce the average measurement integration times for the complete set of calibration parameters by /spl sim/N/sub c/ relative to the corresponding values for single-element calibration procedures. This is significant for satellite systems as calibration must be performed in a short enough time window that the process can be treated as being stationary. Proofs are given that the orthogonal codes satisfy the mathematical lower bounds for the asymptotic forms of calibration parameter estimation variances.

Airborne measurements of total sulfur gases during NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3

A metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) technique was used by investigators from the University of Idaho (UI) to measure ambient total sulfur gas concentrations from an aircraft platform during the NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3 (GTE/CITE 3) program. The MFC/FD/FPD technique allowed rapid quantitation of tropospheric background air masses using sample integration times of 1–3 min with little or no gap between measurements. The rapid and continual sampling nature of this technique yielded data covering approximately 75% of the entire CITE 3 program's air track. Ambient air measurement data obtained during northern hemisphere (NH) flights often exhibited relatively high total sulfur gas values (up to 19 ppb) and an extremely high degree of sample heterogeneity, especially in coastal locations. Data from southern hemisphere (SH) flights typically exhibited relatively low total sulfur gas concentrations and a low degree of sample heterogeneity. A bimodal interhemispheric total sulfur gas gradient was observed using data obtained during transit flights between the two CITE 3 program ground bases. Comparisons were made of UI total sulfur gas measurements with composite sulfur gas values generated using speciated sulfur gas measurements from other CITE 3 participants. Only a relatively small number of overlap periods for comparison were obtained from all the available CITE 3 data because of large differences in measurement integration times and lack of synchronization of sample start/stop times for the various investigators. These effects were compounded with extreme sample heterogeneity in the NH and the speed at which the aircraft traversed the air masses being sampled. Despite these constraints, sufficient overlapping data were available for the comparative evaluations. Comparison of NH UI total with composite sulfur gas values showed excellent correlation and linear curve fit, indicating substantial qualitative agreement. Simple linear regression of total on composite sulfur gas data yielded a slope of 1.9 for coastal NH regions and 1.2 for marine NH regions. The marine NH slope is not statistically different from one, indicating substantial quantitative agreement between UI total and composite sulfur gas values in these regions. However, a significant difference was observed when these same data were treated with a paired t test. SH data exhibited no significant correlation or linear regression slope. A paired t test showed a statistically significant difference when all SH flights were used. However, data from three SH flights that were classified into a unique group using discriminant analysis showed no significant difference between UI total and composite sulfur gas values when analyzed with the paired t test.

Modulation transfer and optical Stark effect in a rubidium atomic clock pumped by a semiconductor laser

Novel spectral line shapes of optical-microwave double resonance with an extremely narrow line width were observed in a rubidium atomic clock pumped by a semiconductor laser. It was confirmed that these novel spectral line shapes of double resonance were due to the modulation-transfer effect. These spectral line shapes were calculated by solving the equation of motion of the density matrix that describes the modulation-transfer process. The calculated results agree with the experimental results. By using the calculated results of the influence of the optical Stark effect on these spectral line shapes, characteristics of the inhomogeneous light shift were evaluated quantitatively for the frequency-modulated microwave. Furthermore, optimum values of modulation parameters were found by computer simulation in order to obtain the highest microwave frequency stability of the atomic clocks. An experiment was carried out employing these optimum parameters, and a short-term frequency stability as high as σy(τ) = 7.9 × 10−13τ−1/2 was obtained, where τ is an integration time of measurement.

Using a 15-μm DFB InGaAsP laser in a passive ring cavity-type fiber gyroscope

It has been demonstrated that a 1.5-microm distributed feedback InGaAsP laser can be used as a coherent light source in a passive ring cavity-type fiber gyroscope. A formula for the detector shot noise-limited sensitivity of detection of rotation was derived for this type of gyroscope. Its value in the present experimental setup was estimated as being 0.8 . tau(-1/2) degree/hour, where tau represents the integration time of measurements. Experiments showed that the uncertainty of the detection of rotation was 1.5 degrees /h at tau = 100 s, which was governed by the slight reflection of light at the AR-coated fiber edges and residual frequency fluctuations of the laser.

Remote Sensing of Atmospheric Temperature Profiles with the Nimbus 5 Microwave Spectrometer

The paper discusses the accuracy of atmospheric vertical temperature profile measurements performed by the Nimbus 5 microwave spectrometer and compares results obtained during the first 6 months of its operation with independent ground-truth measurements (coincident radiosonde measurements). The microwave spectrometer can measure temperature profiles averaged over the instrument weighting function (about 10 km) layers with a root-mean-square accuracy of a few tenths of a degree K for measurement integration times of 16 s. Lower tropospheric temperature data from the spectrometer measurements is used in the numerical weather prediction model of the National Meteorological Center.