Optical Reference Research Articles

Ozonation of 4-aminotoluene as a new method of synthesis of 4-aminobenzaldehyde – an intermediate for the production of anti-tuberculosis drugs

Solutizon is an original anti-TB drug that is effective in resisting mycobacteria to other anti-TB drugs, which is obtained by the interaction of thiosemicarbazone 4-aminobenzaldehyde and sodium oxymethylene sulfonate. 4-Aminobenzaldehyde is synthesized by redox conversion of 4-nitrotoluene in the presence of sodium polysulfide. The reaction is carried out in boiling alcohol, and 4-aminobenzaldehyde is separated after steam distillation with a yield of 40–50 %. However, today this method loses its practical, environmental and economic attractiveness, as it has significant disadvantages – low product yield, high reaction temperature (80–120 °C), the formation of sulfur-containing wastewater. Therefore, the development of low-temperature, environmentally friendly methods for obtaining 4-aminobenzaldehyde is an urgent task. The aim of the work is to study the kinetic features and mechanism of the liquid-phase reaction of ozone with 4-aminotoluene to create a new low-temperature, environmentally friendly method for the synthesis of 4-aminobenzaldehyde. Materials and methods. Sigma acetic anhydride of сh.р. qualification was used for the experiments. 4-Aminotoluene company “Sinbias” qualification “сh.р.”; 4-Acetamidotoluene and its derivatives were used chromatographically pure. Acetates of metals of qualification “сh.р.”, potassium bromide of qualification “pharmacopoeial” were used without additional purification. Continuous control of the current ozone concentration and recording the results in the form of a kinetic curve was carried out when passing ozone-containing gas through the container of the spectrophotometer “SF-46 LOMO” at a certain wavelength of a monochromatic light source. The results of the analysis were recorded using the KSP-4 potentiometer included in the spectrophotometer optical density reference circuit. This device automatically compensated the photocurrent by recording its value. The scale KSP-4 was calibrated in units of optical density, and the conversion into absolute ozone concentration was carried out according to the Lambert-Ber equation using molar extinction coefficients. Relative analysis error ≤5 %. At the optical stroke length of the container 10 ÷ 100 mm, the sensitivity of the device was ~10-7mol·l-1 ozone. Results. The kinetic features and mechanism of the liquid-phase reaction of ozone with 4-aminotoluene have been studied. It is shown that the developed catalytic system Mn(II)-KBr-H2SO4-Ac2O significantly increases the depth, rate, and selectivity of oxidation of 4-aminotoluene and the main reaction product is 4-aminobenzaldehyde in the form of the corresponding benzylidenediacetate with a yield of 69.5 %. The active particle responsible for the inclusion of the substrate in the oxidation of the methyl group in the presence of manganese (II) acetate and potassium bromide is manganese bromide ion radical (Mn(II)Br•), which is more active than Mn (III) and therefore more high-speed initiates oxidation by the methyl group. Conclusions. Catalytic systems have been developed that allow the oxidation of ozone to be directed mainly to the methyl group of 4-aminotoluene and to stop the reaction at different oxidation depths. It was found that manganese (II) acetate, which has a relatively low redox potential, in the system Ac2O-ArCH3-H2SO4-O3 at a temperature of 20 °C has a high substrate selectivity in the reactions of formation of 4-aminobenzyl alcohol. Manganese (II) acetate in the presence of potassium bromide forms a manganese bromide complex with increased catalytic activity, which under the same conditions contributes to the predominant production of 4-aminobenzaldehyde.

Orbiting laser configuration and sky coverage: coherent reference for Breakthrough Starshot ground-based laser array

We present the concept of using an orbiting laser as a coherent optical reference to phase a several kilometer diameter array of ground-based lasers designed to accelerate interstellar nano-spacecraft to 20% light-speed using laser propulsion. We investigate the geometrical and temporal constraints for the initial case of the target star Proxima b in the Alpha Centauri system using a laser ground site in the southern hemisphere. Based on these constraints, we detail requirements for the mission architecture for an orbiting laser to be used as an optical reference. We then present two orbits that can meet all given requirements and represent a range of engagement times and days between engagements. We also present a range of orbits with periods from 3 to 4 days and engagement times from 660 to 800 s. If desired, the orbit can be matched to the sidereal day, so each orbit period, the beacon can align with the ground station and the same target star without maneuvers. A discussion of the tradeoff between the Earth-based site latitude, time on engagement, and days between engagements is presented.

Coherent fibre link for synchronization of delocalized atomic clocks.

Challenging experiments for tests in fundamental physics require highly coherent optical frequency references with suppressed phase noise from hundreds of kHz down to μHz of Fourier frequencies. It can be achieved by remote synchronization of many frequency references interconnected by stabilized optical fibre links. Here we describe the path to realize a delocalized optical frequency reference for spectroscopy of the isomeric state of the nucleus of Thorium-229 atom. This is a prerequisite for the realization of the next generation of an optical clock - the nuclear clock. We present the established 235 km long phase-coherent stabilized cross-border fibre link connecting two delocalized metrology laboratories in Brno and Vienna operating highly-coherent lasers disciplined by active Hydrogen masers through optical frequency combs. A significant part (up to tens of km) of the optical fibre is passing urban combined collectors with a non-negligible level of acoustic interference and temperature changes, which results in a power spectral density of phase noise over 105 rad2· Hz-1. Therefore, we deploy a digital signal processing technique to suppress the fibre phase noise over a wide dynamic range of phase fluctuations. To demonstrate the functionality of the link, we measured the phase noise power spectral density of a remote beat note between two independent lasers, locked to high-finesse stable resonators. Using optical frequency combs at both ends of the link, a long-term fractional frequency stability in the order of 10-15 between local active Hydrogen masers was measured as well. Thanks to this technique, we have achieved reliable operation of the phase-coherent fibre link with fractional stability of 7 × 10-18 in 103 s.

Digital Doppler-Cancellation Servo for Ultrastable Optical Frequency Dissemination Over Fiber.

Progress made in optical references, including ultrastable Fabry-Perot cavities, optical frequency combs, and optical atomic clocks, has driven the need for ultrastable optical fiber networks. Telecom-wavelength ultrapure optical signal transport has been demonstrated on distances ranging from the laboratory scale to the continental scale. In this article, we present a Doppler-cancellation setup based on a digital phase-locked loop (PLL) for ultrastable optical signal dissemination over fiber. The optical phase stabilization setup is based on a usual heterodyne Michelson-interferometer setup, while the software-defined radio (SDR) implementation of the PLL is based on a compact commercial board embedding a field-programmable gate array and analog-to-digital and digital-to-analog converters. Using three different configurations, including an undersampling method, we demonstrate a 20-m-long fiber link with residual fractional frequency instability as low as 10-18 at 1000 s and optical phase noise of -70 dBc/Hz at 1 Hz with a telecom frequency carrier.

10 GHz generation with ultra-low phase noise via the transfer oscillator technique

Coherent frequency division of high-stability optical sources permits the extraction of microwave signals with ultra-low phase noise, enabling their application to systems with stringent timing precision. To date, the highest performance systems have required tight phase stabilization of laboratory grade optical frequency combs to Fabry–Pérot optical reference cavities for faithful optical-to-microwave frequency division. This requirement limits the technology to highly controlled laboratory environments. Here, we employ a transfer oscillator technique, which employs digital and RF analog electronics to coherently suppress additive optical frequency comb noise. This relaxes the stabilization requirements and allows for the extraction of multiple independent microwave outputs from a single comb, while at the same time, permitting low-noise microwave generation from combs with higher noise profiles. Using this method, we transferred the phase stability of two high-finesse optical sources at 1157 and 1070 nm to two independent 10 GHz signals using a single frequency comb. We demonstrated absolute phase noise below −106 dBc/Hz at 1 Hz from the carrier with corresponding 1 s fractional frequency instability below 2 × 10−15. Finally, the latter phase noise levels were attainable for comb linewidths broadened up to 2 MHz, demonstrating the potential for out-of-lab use with low SWaP lasers.

Limitations due to residual interference in a fiber-based optical frequency reference at 1.55 µm

We present an experimental investigation of the stability limits specific to optical frequency standards using fiber optic architecture and semiconductor lasers. A compact setup composed of a semiconductor laser frequency-locked onto an acetylene transition detected in saturated absorption has been implemented using only fiber optic components. Fiber optic technology allows compact and reliable solutions for various applications. However, for high sensitivity and stability applications such as metrology, residual reflections induced by optical index inhomogeneities in connectors and fiber-coupled components leading to spurious interference significantly limit performance. We have examined the origin of the interference fringes superimposed on the detected signal and the limitations they cause to the frequency stability of the reference. The effects of temperature and beam power fluctuations are also examined. Our results show that the frequency stability is limited in the 10 − 13 range by the effect of interference fringes due to use of fiber components.

Automatic calving front extraction from digital elevation model-derived data

Calving Front (CF) is an important parameter to analyse ice sheet dynamics and to measure glacier mass balance. DEM products covering polar regions are critical remote sensing data sources to provide fundamental terrain information for glaciological studies. It is necessary to provide accurate CFs for DEM specific applications, such as mass balance calculation or the substitution of water values with geoid values in the DEM editing process. However, much less attention is paid to automatically delineate CFs from DEM data. In this study, we propose a DEM-based Automatic CF Extraction method (DACE) to efficiently extract the CFs from DEM data. In DACE, a height-sensitive terrain feature is designed to enhance the contrast between the ice sheet and the ocean by combining elevation and roughness information. To improve the CF extraction performance, a two-category image classification based on game theory is proposed that considers the spatial consistency of the feature image created. For validation, DACE was applied to DEM products generated from the single-pass SAR interferometry mission TanDEM-X (TDM) at different posting sizes (12 and 90 m) and to the optical photogrammetry-based Reference Elevation Model of Antarctica (REMA) with a 2 m posting. The proposed algorithm can achieve a CF extraction accuracy of better than 14, 20, and 70 m for the 2-m REMA, 12- and 90-m TDM DEMs, respectively, when compared with the manually delineated CFs. The experimental results demonstrate that the proposed algorithm can effectively extract the CFs from DEM data. DACE can be used to replace water values and edit the DEMs themselves. The CFs extracted from the DEM data can also be used for glacier mass balance calculation with the DEM-based geodetic method and for the temporal analysis of CF changes when multi-temporal DEM data are available.

Efficient application of optical references in surveying

Efficient application of optical references in surveying

Calibration of ISA accelerometer sensing axes for the BepiColombo mission

ISA is a three-axis accelerometer embarked on-board BepiColombo, a joint ESA/JAXA space mission launched on the 19th of October 2018 ​at 22.45 (local time) and devoted to put two spacecraft (MPO and MMO) around Mercury (2025), for a deeper investigation of the planet and its environment. ISA aims at measuring, directly and not by modelling, first time for a planet other than Earth, the non-gravitational perturbations the MPO will undergo during its stay around Mercury. Such data will be used within the frame of the Radio-Science Experiments to reconstruct, a posteriori and with very high accuracy, the true gravitational orbit of the spacecraft and, at last, the gravity field of Mercury. Three single accelerometers, each one characterised by its own sensing axis, i.e. the direction along which it is sensitive to accelerations, collect the non-gravitational perturbations encountered by the MPO. The arrangement of sensing axes, whose knowledge is essential to reconstruct the sensed accelerations, was measured with very high accuracy by the Experimental Gravitation group at IAPS/INAF in Rome, which manages ISA and holds the PI-ship. The group developed a calibration procedure to identify on-ground the direction of ISA sensing axes with respect to an optical reference frame. An experimental facility was designed and assembled to accomplish the measurement. In November 2015, the calibration was performed on the ISA Flight Model, currently in cruise phase towards Mercury. Results of the measurement campaign are reported and prove the determination of ISA sensing axes with an accuracy within 20–30 arcseconds per single axis, well below the 100 arcseconds required by the Radio-Science needs.

Direct-determination of high-concentration sulfate by serial differential spectrophotometry with multiple optical pathlengths

Direct-determination is a fast and free of contamination method for analysis of substances in water samples. However, direct-determination of SO42− with high concentration in environmental systems is still challenging due to deviation from Beer-Lambert law is generally observed when a substance with high concentration is directly determined, resulting in poor accuracy, sensitivity, and narrow linear range. In this study, a simple and rapid method for the direct-determination of SO42− with high concentration was proposed. Serial high-absorbance differential spectrophotometry was applied to incrementally widen the determination range under different optical pathlengths. In this process, the effects of optical pathlength and reference concentration on sensitivity were further investigated. The results showed that SO42− could be accurately quantified within a concentration range of 0–4.10 g/L, and the determination range by this method was 10-fold and 19.5-fold wider than those by conventional differential spectrophotometry and conventional spectrophotometry, respectively. And the applicable ranges of sensitivity were obtained at various optical pathlengths by adjusting the reference solution concentration. This approach exhibited several advantages over conventional methods, including high accuracy, excellent precision, low cost, less time consumption, and easy operation. This method is promising and can provide accurate and reliable data support for environmental monitoring and pollution control.

Assessment of the retinal vasculature in healthy Chinese preschool children aged 4\u20136\u2009years old using optical coherence tomography angiography

PurposeTo establish normal parameters of macular and optic disc vasculature by optical coherence tomography angiography (OCTA) in healthy preschool children aged 4–6 years old in China. OCTA reflects retinal metabolism and development in children at these ages and could be used clinically and in future studies to aid diagnosis and prediction of retinal abnormalities and developmental stagnation.MethodsIn this cross-sectional study, we measured foveal, parafoveal, and perifoveal vessel density in the superficial capillary plexus (SCP); the deep capillary plexus (DCP), the foveal avascular zone (FAZ), and the radial capillary peripapillary (RPC) in the optic disc using investigational spectral-domain OCTA. The magnification effect of the FAZ area and microvasculature measurements was corrected by Littman and the modified Bennett formula.ResultsA total of 242 eyes (116 males and 126 females, 5.31 ± 0.73 years) were recruited for the analysis. The mean macular vessel density was 48.10 ± 2.92% and 48.74 ± 6.51% in the SCP and the DCP, respectively. The RPC vessel density was 47.17 ± 2.52%, 47.99 ± 4.48%, and 48.41 ± 3.07% in the whole image, inside disc, and peripapillary, respectively; and the mean FAZ area was 0.28 ± 0.11 mm2. A significant difference between male and female participants was found in the retinal vasculature (DCP, SCP, and RPC). None of these parameters were significantly different in age (P > 0.05), except that DCP slightly increased with aging. The right and left eyes had good consistency in the parameters of the macula and optic disc.ConclusionsOur study establishes the macular and optic disc OCTA reference values in 4- to 6-year-old healthy preschool children. They may be used in longitudinal OCTA studies and clinical applications.

Tellurium‐stabilized blue laser diode

AbstractA robust and compact blue optical frequency reference within blue region was designed. A 40‐mW tellurium‐stabilized single‐frequency blue diode laser in external cavity was designed with an absorption spectroscopy method based on tellurium molecular transitions. Furthermore, the uncertainty of the frequency stabilization can be pushed to 2 × 10−9 at a 10‐s integration time. In addition, the correspondence frequency instability was 1.3 MHz when the laser wavelength was 445.75 nm. This study indicates that high‐stability frequency reference within the blue region can be achieved with an external cavity diode laser as opposed to the diode‐pumped frequency‐doubled solid‐state laser.

Calibration of piezoresistive shape-memory alloy strain sensors

Continuous strain measurement on fibre-reinforced structures demands mechanical sensors with superior fatigue resistance. Shape-memory alloy wires are predestined for strain sensors utilising their strong piezo-resistance. Calibration of these sensors is necessary in order to extract mechanical data. Therefore, four-point bending of glass-fibre reinforced plastic specimens with applied strain sensors and an optical reference measuring system is used to calibrate and compare shape-memory alloy sensors and standard strain gauges. The gauge factor and its standard deviation is successfully measured by this calibration method. Shape-memory alloy sensors show strain-dependent gauge factor whilst standard strain gauges show a constant strain sensitivity, both with a narrow stochastic distribution. Shape-memory alloy mechanical sensors are reliable to determine strain of fibre-reinforced structures. This offers the possibility to use them in structural health monitoring applications of such structures. Consequently, the four-point bending calibration using glass-fibre reinforced specimens represents a suitable possibility for calibration of strain sensors exposed to higher strain amplitudes.

Noise Reduction Method of Φ-OTDR System Based on EMD-TFPF Algorithm

The phase-sensitive optical time domain reflectometry ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR) system has been gradually applied to vertical seismic profile exploration due to its excellent anti-electromagnetic interference, extremely high resolution and sensitivity, and wide detection range. However, the actual data is often disturbed by random noise, which seriously affects the system’s ability to recognize low-frequency disturbances. In order to suppress the background noise and improve the signal-to-noise ratio (SNR), a fusion noise reduction method based on empirical mode decomposition and time-frequency peak filtering (EMD-TFPF) algorithm is studied and introduced. In terms of seismic exploration data processing, the algorithm has good denoising performance. Using the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR system based on optical synchronous reference heterodyne detection as the experimental platform, the EMD-TFPF noise reduction algorithm is applied to the position information and compared with the VSS-NLMS, VMD-NWT and EMD-soft methods. Experimental results show that under the conditions of 0.1 Hz and 5 V vibration interference, the EMD-TFPF method improves the SNR to 37.6 dB, which is much higher than the other three noise reduction algorithms. In addition, this method broadens the low-frequency response range of the system to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> Hz. The improvement of the system’s ability to recognize low-frequency disturbance events will undoubtedly accelerate the practical process of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR system in seismic exploration.

Electro-optically derived millimeter-wave sources with phase and amplitude control

Integrated circuits are building blocks in millimeter-wave handsets and base stations, requiring nonlinear characterization to optimize performance and energy efficiency. Today's sources use digital-to-analog converters to synthesize arbitrary electrical waveforms for nonlinear characterization, but this approach demands even faster integrated circuits to increase the bandwidth to millimeter-waves. Optically derived sources are a potential path to generate precise millimeter-waves and arbitrary waveforms using additive frequency synthesis. In this work, we demonstrate optically derived millimeter-waves up to 99.2 GHz with phase and amplitude control that could be locked to an optical reference. Our approach uses a 1550 nm electro-optic frequency comb with a terahertz of bandwidth. A programmable spectral filter selects two wavelengths from the optical comb, illuminating a modified uni-traveling carrier photodiode on a coplanar waveguide. We then tune the phase and amplitude by varying the optical phase and amplitude in the programmable spectral filter. The result of our work is electro-optically derived millimeter-waves at (24.8, 49.6, 74.4, and 99.2) GHz with phase and amplitude control, enabling arbitrary repetitive waveform generation.

Complex Kernel-based spectrum reconstruction algorithm for cascaded Fabry-Perot interferometric spectrometer.

A method to calculate the spectrum of the light incident on a cascaded Fabry-Perot interferometric spectrometer from the detector signal versus the scanning mirror position is presented. The method is based on modifying the Fabry-Perot integral equation to reduce possible spectrum reconstruction errors that arise due to inaccurate determination of the optical path difference reference position and the dependence on the dispersion of the cavity material. A transformation algorithm that employs the suggested kernel modification is derived and tested. The presented algorithm is then compared to the conventional kernel, showing spectral error reductions by larger than 20 dB.

Estimation of reducing the impact of the optical lattice spectrum on the optical clock frequency shift

In this paper the method for reducing the shift of the “clock” resonance by the example of 87Sr has been described. This shift arises due to the presence in the optical spectrum of the lattice laser of the incoherent component. This component is a traveling wave. It is shown that the effective decrease of the impact on the shift of the “clock” resonance by introducing the interference filter is achieved. This filter was inserted directly into the optical scheme after the laser source. Examples of the optical lattice spectra with and without filter are considered. The estimates of the efficiency ratio of the use of the interference filters to improve the final uncertainty of the optical reference are made. It is shown that the filter reduces the relative uncertainty from the impact of the incoherent component of the optical radiation of the optical lattice by two orders of magnitude. It should help to reduce the lower bound for the total uncertainty limit of the optical frequency clocks down to 10−18.

Optical frequency comb stabilized to a fiber delay line

We have developed a simple and practical optical frequency comb (OFC) stabilization method for comb linewidth reduction in this Letter. Two comb modes of an Er-fiber optical frequency comb (OFC) have been phase-locked to a kilometer-long fiber delay line, while narrow linewidth lasers are not required as optical references. The fractional frequency stability of the 1542-nm comb mode in the Er-fiber OFC reaches 9.13 × 10−13 at 12.8 ms average time, and its short-term linewidth is 580 Hz, which is compressed by a factor of ∼170 compared to the free-running condition. The whole stabilization scheme gets rid of nonlinear progress, which can be an alternative approach for OFC stabilization, especially in ultra-high repetition-rate combs, e.g., electro-optic combs, quantum cascade laser combs, and micro-combs with low pulse energy.

Simultaneous detection of the distance and direction for a noncooperative target based on the microwave photonic radar.

For a noncooperative target, multiple-dimension detection is required to determine its spatial location. A microwave photonic radar is proposed for simultaneous detection of the distance and direction of targets. At the transmitter, a linear frequency modulation (LFM) signal with a large instantaneous bandwidth was obtained by frequency doubling. Meanwhile, the optical reference signal was supplied. At the receiver, two antennas with a certain baseline length were used to receive the echo signal. The dechirp process and fixed-length cable are combined to decouple the distance and direction and distinguish between the positive and negative directions of a target. Theoretical derivation has already verified that the distance and direction can be resolved simultaneously by dechirping of the echo modulation signal and reference signal. In experiments, an LFM signal with an instantaneous bandwidth of 2 GHz (2-4 GHz) was generated and launched as the detection signal. Targets at different distances with positive and negative angles were detected. The results show that the directional measurement ranges from -72.5° to 72.5°, and the error is less than 1.6°. The measured distance was found to be quite similar to the actual distance. The proposed approach can confirm the spatial location of a noncooperative target by combining the detection of both distance and direction. Furthermore, the baseline length need not be less than half the wavelength of the echo signal, and large antennas can be chosen to improve the detection distance and range accuracy.

Single step zero-thermal-expansion temperature measurement of optical reference cavities.

State-of-the-art laser frequency stability has been pushed to the 10-17 level. The laser reference cavity is typically nested in a multi-layer thermal enclosure to increase vacuum thermal time constant and thermally controlled at the zero-thermal-expansion temperature to reduce the external temperature fluctuation effect. It is rather time consuming to accurately determine the zero-thermal-expansion temperature for a large thermal time constant system. Here we develop a fast method for measuring the zero-thermal-expansion temperature of the cavity by relying on just one single temperature scan. We first develop a theoretical model to predict the performance of the laser locked to the reference cavity, and then construct an evaluation system for verification of the model. The zero-thermal-expansion temperature of a 30-cm cavity is measured to be 4.3±0.5°C. The fast and high precision method for determining the zero-thermal-expansion temperature will be valuable in improving long-term frequency stabilities of cavity stabilized lasers.