Wavelength-dependent Time Delay Research Articles

Spectroscopic photoacoustic microscopic imaging during single spatial scan using broadband excitation light pulses with wavelength-dependent time delay

In most multispectral optical-resolution photoacoustic microscopy (OR-PAM), spatial scanning is repeated for each excitation wavelength, which decreases throughput and causes motion artifacts during spectral processing. This study proposes a new spectroscopic OR-PAM technique to acquire information on the photoacoustic signal intensity and excitation wavelength from single spatial scans. The technique involves irradiating an imaging target with two broadband optical pulses with and without wavelength-dependent time delays. The excitation wavelength of the sample is then calculated by measuring the time delay between the photoacoustic signals generated by the two optical pulses. This technique is validated by measuring the excitation wavelengths of dyes in tubes. Furthermore, we demonstrate the three-dimensional spectroscopic OR-PAM of cells stained with suitable dyes. Although the tradeoff between excitation efficiency and excitation bandwidth must be adjusted based on the application, combining the proposed technique with fast spatial scanning methods can significantly contribute to recent OR-PAM applications, such as monitoring quick biological events and microscale tracking of moving materials.

Quantum-inspired detection for spectral domain optical coherence tomography.

Intensity levels allowed by safety standards (ICNIRP or ANSI) limit the amount of light that can be used in a clinical setting to image highly scattering or absorptive tissues with optical coherence tomography (OCT). To achieve high-sensitivity imaging at low intensity levels, we adapt a detection scheme-which is used in quantum optics for providing information about spectral correlations of photons-into a standard spectral domain OCT system. This detection scheme is based on the concept of dispersive Fourier transformation, where a fiber introduces a wavelength-dependent time delay measured by a single-pixel detector, usually a high-speed photoreceiver. Here, we use a fast superconducting single-photon detector SSPD as a single-pixel detector and obtain images of a glass stack and a slice of onion at the intensity levels of the order of 10 pW. We also provide a formula for a depth-dependent sensitivity falloff in such a detection scheme, which can be treated as a temporal equivalent of diffraction-grating-based spectrometers.

Precision Dynamic Sensing With Ultra-Weak Fiber Bragg Grating Arrays by Wavelength to Frequency Transform

A high-speed interrogator using wavelength to frequency transform is proposed for high-precision demodulation of ultra-weak fiber Bragg gratings (UWFBGs). An amplified spontaneous emission source, modulated by a frequency-swept microwave signal, is adopted as the light source. The signal reflected by the UWFBGs is temporally broadened by a dispersion element, which is then detected and mixed with the driven microwave signal. Through wavelength-dependent time delay introduced by the dispersion element and the frequency sweeping of the microwave signal, the wavelength of the fiber Bragg grating is converted to a beat frequency by the mixing process. Due to the ambient temperature fluctuation, the effective refractive index of the sensing fiber varies over time. Consequently, the induced time delay varies with temperature, generating errors in the wavelength demodulation. By using a dispersion compensating fiber and a single-mode fiber, which introduce opposite group-velocity dispersion (GVD), such error is effectively reduced. Moreover, a dispersion correction model, which considers the wavelength dependence of GVD, is established to replace the traditional dispersion model. Experimental demonstration verifies that the interrogator, operating at 80 kHz, shows high wavelength demodulation accuracy of 6.96 pm.

A Novel Photonic-Based MIMO Radar Architecture With All Channels Sharing a Single Transceiver

A novel photonic-based multiple-input multiple-output (MIMO) radar architecture is reported based on optical frequency combs and dispersion induced wavelength-dependent time delay. Using the proposed system architecture, we only need one photonic-based radar waveform generator at the transmitter side, whereas only a single receiver, consisting of a photodetector, a low-pass filter and an analog-to-digital convertor, is required at the receiver side. The key significance of the work is that all channels of the MIMO radar system share a single photonic-based transceiver, which can solve the bottleneck problem that has long restricted the application of MIMO radar with a large-scale antenna array, i.e., each channel in a MIMO radar system needs an independent transmitter and an independent receiver. The proposed photonic-based MIMO radar architecture is analyzed, demonstrated and verified by simulation using a $10\times10$ MIMO radar system and a $15\times15$ MIMO radar system. The maximum MIMO radar array size that can be supported by the system architecture is also discussed, and constraints on the number of transmitting antennas and receiving antennas are theoretically given.

High-Speed Distributed Sensing Based on Ultra Weak FBGs and Chromatic Dispersion

A high-speed quasi-distributed fiber optic sensing technique is proposed. A sensing link consisting multiple identical fiber Bragg gratings (FBGs) was interrogated by a pulsed light source, and a dispersive unit at the receiver end was used to convert the wavelength shifts of the reflected pulses into time-delay changes. This Bragg wavelength-dependent time delay can be accurately calculated by a fast cross-correlation-based algorithm, enabling distributed high-speed sensing. This technique is experimentally demonstrated using a sensing link containing 2014 FBGs with a reflectance of about −35 dB. A wavelength resolution of 9.03 pm was achieved without any data averaging. Dynamic strain was applied to one of the FBGs, and corresponding wavelength variation was measured at a sampling rate of 20 kHz, obtaining a dynamic resolution of $\surd $ Hz.

Violent intranight optical variability of the blazar S4 0954+65 during its unprecedented 2015 February outburst

Abstract In this letter, we present results from intranight monitoring in three colours (BRI) of the blazar S4 0954+65 during its recent (2015 February) unprecedented high state. We find violent variations on very short time-scales, reaching magnitude change levels of 0.1–0.2 mag h−1. On some occasions, changes of ∼0.1 mag are observed even within ∼10 min. During the night of 2015 February 14, an exponential drop of ∼0.7 mag is detected for about 5 h. Cross-correlation between the light curves does not reveal any detectable wavelength-dependent time delays, larger than ∼5 min. Colour changes ‘bluer-when-brighter’ are observed on longer time-scales. Possible variability mechanisms to explain the observations are discussed and a preference to the geometrical one is given.

반사형 반도체 광 증폭기를 이용한 음계수를 가지는 광 마이크로파 노치 필터

We propose and experimentally demonstrate a photonic microwave notch filter with a negative coefficient. The negative coefficient is obtained by using cross gain modulation (XGM) in a reflective semiconductor optical amplifier (RSOA). When the RSOA is operated in saturated region, the signal carried on the pump wavelength is inversely copied to the probe wavelength by the XGM effect, showing a negative coefficient. Time delay between pump signal and probe signal is provided by single mode fiber (SMF) with wavelength dependent time delay. The proposed microwave notch filter shows notch dips of more than 35.1 dB and free spectral range (FSR) of about 380.6 MHz. 논문 14-39C-05-03 The Journal of Korea Information and Communications Society '14-05 Vol.39C No.05 http://dx.doi.org/10.7840/kics.2014.39C.5.418 418 ※ 본 연구는 산업통산자원부 산하 (재)충청지역사업평가원 융합전자기기 선도사업 신호처리융합기기 프로젝트의 지원을 받아 수행된 연구임. ※ 이 논문은 2013년도 정부(교육부)의 재원으로 한국연구재단의 지원을 받아 수행된 기초연구사업임(NRF-2009-0093828). First Author : School of Information and Communications, Gwangju Institute of Science and Technology, wbkwon@gist.ac.kr, 정회원 ° Corresponding Author : School of Information and Communications, Gwangju Institute of Science and Technology, csp@gist.ac.kr, 정회원 * 광주과학기술원 정보통신공학부, cyk@gist.ac.kr. 정회원 ** 목포대학교, srlee@mokpo.ac.kr. 종신회원 논문번호:KICS2014-02-030, Received Feburary 3, 2014; Revised April 17, 2014; Accepted April 17, 2014 논문 / 반사형 반도체 광 증폭기를 이용한 음계수를 가지는 광 마이크로파 노치 필터

Tunable coherence-free microwave photonic bandpass filter based on double cross gain modulation technique

A tunable, coherence-free, high-resolution microwave photonic bandpass filter, which is compatible to be inserted in a conventional fiber optic link, is presented. It is based on using two cross gain modulation based wavelength converters in a recursive loop. The double cross gain modulation technique solves the semiconductor optical amplifier facet reflection problem in the conventional recursive structure; hence the new microwave photonic signal processor has no coherent interference and no phase-induced intensity noise. It allows arbitrary narrow-linewidth telecommunication-type lasers to be used while enabling stable filter operation to be realized. The filter passband frequency can be tuned by using a wavelength tunable laser and a wavelength dependent time delay component. Experimental results demonstrate robust high-resolution bandpass filter operation with narrow-linewidth sources, no phase-induced intensity noise and a high signal-to-noise ratio performance. Tunable coherence-free operation of the high-resolution bandpass filter is also demonstrated.

Intrinsic quasar variability and time delay determination in the lensed quasar UM673

We present the results of time delay measurements between the A and B components of the gravitationally lensed quasar UM673. The measurements are based on the brightness variations of the quasar components observed with the 1.5-m telescope of the Maidanak Observatory in the V, R and I bands during the 2003–2005 observational period. The amplitudes of variations in the R band are 0.046 and 0.122 mag for Component A and B, respectively. Due to the short visibility period of UM673, about 3 months in a year, the light curves of the quasar components have large annual gaps. It complicates the time delay estimation. From the cross-correlation analysis, we find that the brightness variations in component B follow the brightness variations in Component A by about 150 days. We also explore the possibility of measuring the wavelength-dependent time delay between the brightness variations in the V, R and I bands. We find that in agreement with reprocessing model of quasar variability, the brightness variation at longer wavelengths follows the brightness variations at shorter wavelengths. Finally, we show that the combination of the multiband data corrected for the wavelength-dependent time delay can help to improve the cross-correlation analysis of the light curves with large annual gaps.

Integrated 10 Gb/s AWG-based correlator for multi-wavelength optical header recognition

In this paper we experimentally demonstrate a novel optical correlator employing dual integrated Arrayed Waveguide Grating (AWG) in conjunction with variable delay lines. The variable delay lines provide wavelength-dependent time delays and generate a wavelength profile that matches arbitrary bit patterns, whereas the AWGs are used to demultiplex and multiplex the wavelength components of the multi-wavelength header bit pattern. The recognition of 4-bit optical patterns at different wavelengths is experimentally demonstrated at 10 Gb/s by showing that the correlator produces an autocorrelation waveform of high peak whenever the input bit pattern matches the wavelengths profile, and a low-amplitude cross-correlation function otherwise.

Testing thermal reprocessing in active galactic nuclei accretion discs

The thermal reprocessing hypothesis in active galactic nuclei (AGN), where extreme ultraviolet/X-ray photons are reprocessed by the accretion disc into optical/UV photons, predicts wavelength-dependent time-delays between the optical continuum at different wavelengths. Recent photometric monitoring by Sergeev et al. has shown that the time-delay is observed in 14 AGN, and generally seen to increase with increasing wavelength, as predicted in the reprocessing scenario. We fit the observed time-delays and optical spectral energy distribution using a disc reprocessing model. The model delivers estimates for the nuclear reddening, the product of black hole mass and accretion rate, and the distance to each object. However, the distances at face value give H0 = 44 ± 5k m s −1 Mpc −1 ‐ a factor of 1.6 smaller than generally accepted. We discuss the implications of this on the reprocessing model.

Opto-VLSI-based correlator architecture for multiwavelength optical header recognition

A novel optical correlator employing an opto-very-large-scale-integration (VLSI) processor to construct the routing lookup table, in conjunction with an array of fiber Bragg gratings (FBGs) for multiwavelength optical header recognition is demonstrated. The FBG array provides wavelength-dependent time delays, whereas the opto-VLSI processor generates wavelength intensity profiles that match arbitrary bit patterns. The recognition of 4-b optical patterns is experimentally demonstrated at 2.2 Gb/s by showing that the correlator produces an autocorrelation waveform of high peak whenever the input bit pattern matches the wavelength intensity profile.

Determining time delay in the gravitationally lensed system QSO2237+0305

Aims. We considered the possibility of measuring time delays between components of the multiplied quasar QSO2237+0305 and between V and R band variations. Methods. The analysis is based on the flux variations of four components observed by the OGLE collaboration and Maidanak group during the last quarter of 2003. The observed gradients of the brightness variations in this period for OGLE data in V band are 3.4 mmag/day, 2.2 mmag/day, 2.4 mmag/day, and 1.1 mmag/day for the components A, B, C, and D, respectively. The variations are probably intrinsic source variations. The basic method used for time-delay measurements is the cross-correlation technique. To estimate the uncertainty of the time-delay measurements, Monte Carlo simulations were carried out. Results. The calculations showed the impossibility of unambiguously measuring the differential time delays between the components. The observations at shorter wavelengths seem to be the only way to achieve robust time delay measurements in the system. The wavelength-dependent time delays can be used to provide the evidence of an accretion-disk structure of the central optical variable source. The observed V and R band variations of component C show good correlation with the correlation coefficient of 0.83. However, the obtained time delay, about 16.2 days, and its accuracy are far from reliable.

High-speed optical bit-pattern recognition employing fibre Bragg grating and Opto-VLSI processing

We propose a novel optical bit-pattern recognition employing an Opto-VLSI processor in conjunction with an array of fibre Bragg gratings (FBGs) with different Bragg wavelengths and a coherent-to-incoherent light converter. The FBG array slices the spectrum of the incoherent optical header and provides wavelength-dependent time delays, whereas the Opto-VLSI processor generates wavelength intensity profiles that match specific bit patterns. The recognition of 8-bit optical patterns is experimentally demonstrated at 2.2 Gb/s by showing that the correlator produces an autocorrelation waveform of high peak whenever the input bit pattern matches the wavelength intensity profile.

Femtosecond optical Kerr-gate measurement of fluorescence spectra of dye solutions

Utilizing the ultrafast electronic response of Kerr effect of glass material as an optical gate, we have constructed a system for the time-resolved fluorescence measurement in the 100-fs time region. We have demonstrated that the ultrafast Kerr-gate technique combined with multi-channel spectral detection enables us to the direct measurement of the spectral shape. Because of highly efficient gating and simultaneous acquisition of all the spectral components, the time-resolved fluorescence spectra of high S/N ratio have been obtained for several dye solutions. Although wavelength-dependent time delay of the system is found to remain, direct recognition of the dynamic Stokes shift in the S 2→S 0 fluorescence of malachite green in ethanol has been made by comparing the obtained time-resolved spectra with those of the ultrafast fluorescence of β-carotene in hexane.

A new direct method for measuring the Hubble constant from reverberating accretion discs in active galaxies

We show how wavelength-dependent time delays between continuum flux variations of active galactic nuclei (AGNs) can be used to test the standard black hole—accretion disc paradigm, by measuring the temperature structure T(R) of the gaseous material surrounding the purported black hole. Reprocessing of high-energy radiation in a steady-state blackbody accretion disc with T ∝ R−3/4 incurs a wavelength-dependent light travel time delay τ∝λ4/3. The International AGN Watch multiwavelength monitoring campaign on NGC 7469 showed optical continuum variations lagging behind those in the UV by about 1 d at 4800 Å and about 2 d at 7500 Å. These UV/optical continuum lags imply a radial temperature structure T ∝ R−3/4, consistent with the classical accretion disc model, and hence strongly support the existence of a disc in this system. We assume that the observed time delays are indeed caused by a classical accretion disc structure, and derive a redshift-independent luminosity distance to NGC 7469. The luminosity distance allows us to estimate a Hubble constant of H0 (cos i/0.7)1/2 = 42 ± 9 km s−1 Mpc−1. The interpretation of the observed time delays and spectral energy distribution in the context of an accretion disc structure requires further validation. At the same time, efforts to minimize the systematic uncertainties in our method to derive a more accurate measurement of H0, e.g. by obtaining an independent accurate determination of the disc inclination i or statistical average of a moderate sample of active galaxies, are required. However, this remains a promising new method of determining redshift-independent distances to AGNs.

Measurement and fitting of spectrum and pulse shapes of a liquid methane moderator at IPNS

The absolute intensity, energy spectrum and energy variation of the neutron pulse shape have been measured for the IPNS liquid methane “F” moderator, at 108 K. The spectrum has been corrected for attenuation by aluminum in the beam and has been fitted using a simple parametric description. The pulse shapes have been fitted to a function which is the sum of two decaying exponentials convoluted with a Gaussian. These shapes have been corrected for instrumental resolution, parameterised as a function of neutron wavelength, and compared to the results of Monte Carlo simulations for equivalent moderator geometry. Only four parameters are required to describe the normalised spectrum and, in addition to wavelength dependent time delays, only five wavelength independent parameters are needed to describe the variation of the pulse shape over the entire spectrum.

Observations of multiple seismic events

abstract Two or more dispersed wave trains each with constant amplitude will interfere giving a resultant wave train which is amplitude modulated, if the individual waves have their principal energies in a common frequency band and if the trains arrive with time separations small compared to their total length. The dispersive characteristics of the trains need not be the same. If the component trains are of comparable magnitude, the modulation due to interference becomes significant and a “beat” phenomenon occurs. Multiple trains of dispersed seismic surface waves may occur because of a temporal and/or spatial distribution at the source or because of multipath propagation. Each of these causal mechanisms influences the amplitude and phase spectra of the resultant wave train; derived properties such as phase velocities and amplitude ratios are also influenced. In the case of multipath propagation, wavelength dependent time delays may occur. Two cases of twin earthquakes are analyzed, and the significant features of interference are demonstrated. In one case, estimates are obtained for the amplitude ratio and time delay of the second shock with respect to the first. The interpretation of seismograms and spectra influenced by multiple events is discussed.