Atmospheric Phase Noise Research Articles

Frequency comb-to-comb stabilization over a 1.3-km free-space atmospheric optical link

Stabilizing a frequency comb to an ultra-stable optical frequency reference requires a multitude of optoelectronic peripherals that have to operate under strict ambient control. Meanwhile, the frequency comb-to-comb stabilization aims to synchronize a slave comb to a well-established master comb with a substantial saving in required equipment and efforts. Here, we report an utmost case of frequency comb-to-comb stabilization made through a 1.3 km free-space optical (FSO) link by coherent transfer of two separate comb lines along with a feedback suppression control of atmospheric phase noise. The FSO link offers a transfer stability of 1.7 × 10–15 at 0.1 s averaging, while transporting the master comb’s stability of 1.2 × 10–15 at 1.0 s over the entire spectrum of the slave comb. Our remote comb-to-comb stabilization is intended to expedite diverse long-distance ground-to-ground or ground-to-satellite applications; as demonstrated here for broadband molecular spectroscopy over a 6 THz bandwidth as well as ultra-stable microwaves generation with phase noise of -80 dBc Hz–1 at 1 Hz.

Mitigation of time-series InSAR turbulent atmospheric phase noise: A review

Synthetic Aperture Radar (SAR) interferometry is one of the most powerful remote sensing tools for ground deformation detection. However, tropospheric delay greatly limits the measurement accuracy of the InSAR technique. While vertically stratified tropospheric delays have been extensively investigated and well tackled, turbulent tropospheric phase noise still remains an intractable issue. In recent years, great efforts have been made to reduce the influence of turbulent atmospheric delay. This contribution is intended to provide a systematic review of the progress achieved in this field. First, it introduces the physical characteristics of atmospheric signals in interferograms. Then, a review of the main mitigation algorithms proposed in the literature is provided. In addition, the strengths and weaknesses of each approach are analyzed to provide guidance for choosing a suitable method accordingly. Finally, suggestions for resolving the challenging issues and an outlook for future research are given.

Deformation Velocity-Based Regularization of Multibaseline SAR Interferometry

Synthetic aperture radar (SAR) interferometry (InSAR) has shown great potential in the monitoring of Earth's surface and detection of the possible slow temporal deformations. Within the framework of multibaseline SAR interferometry, the availability of multiple interferograms obtained from multipass satellite observations can significantly improve the accuracy of the estimated target parameters, i.e., the residual height and the mean deformation velocity. The parameters can be estimated in the maximum likelihood (ML) sense and through the data covariance matrix. However, the presence of artifact and outliers may impair the parameter estimation, specifically when the candidate cells are subject to temporal decorrelation and atmospheric phase noise effects. In this letter, the exploitation of contextual spatial information is proposed to reduce the possible ambiguity and improve the accuracy of ML-based parameter estimation. The proposed approach adds a regularization term (or a constraint) to the ML's model in order to include the information about the scene velocity variation. Hence, the resulted nonconvex optimization is resolved using the graph-cut concept. The method is evaluated using the simulated and two real data sets acquired by Constellation of Small Satellites for Mediterranean basin Observation (COSMO-SkyMed) and Sentinel-1A sensors over Tehran, Iran; and the results are validated using the global positioning system-based measurements.

Phase-stabilized free-space link for optical frequency transfer

Optical frequency transfer through the atmosphere is needed for diverse free-space applications, e.g. coherent communication, optical clock comparison and relativity experiments. The task requires an appropriate optical link system that provides the multimodal abilities of not only pointing, acquisition, and tracking (PAT) but also phase noise suppression in the presence of air turbulence. Here, we describe a free-space optical (FSO) link system built over a 1.4 km outdoor path to transmit optical frequencies with a fast beam tracking capability together with an active feedback compensation scheme of the atmospheric phase noise. The FSO link is able to offer a frequency transfer stability of 3.51 × 10−19 at 100 s averaging, which suffices to deliver the current state-of-the-art optical clock signals.

Ultra-Wideband Free-Space Optical Phase Stabilization

Free-space optical (FSO) communications have the potential to revolutionize wireless communications due to the advantages of greater inherent security, high-directionality, high available bandwidth and small physical footprint. The effects of atmospheric turbulence currently limit the performance of FSO communications. In this letter, we demonstrate a system capable of indiscriminately suppressing the atmospheric phase noise encountered by independent optical signals spread over a range of 7.2 THz (encompassing the full optical C-Band), by actively phase stabilizing a primary optical signal at 193.1 THz (1553 nm). We show ~30 dB of indiscriminate phase stabilization over the full range, down to average phase noise at 10 Hz of -39.6 dBc Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> when using an acousto-optic modulator (AOM) as a Doppler actuator, and -39.9 dBc Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> when using a fiber-stretcher as group-delay actuator to provide the phase-stabilization system's feedback. We demonstrate that this suppression is limited by the noise of the independent optical signals, and that the expected achievable suppression is over 40 dB greater, reaching around -90 dB Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 10 Hz. We conclude that 40 Tbps ground-to-space FSO transmission would be made possible with the combination of our stabilization system and other demonstrated technologies.

Methods for coherent optical Doppler orbitography

Doppler orbitography uses the Doppler shift in a transmitted signal to determine the orbital parameters of satellites including range and range rate (or radial velocity). We describe two techniques for atmospheric-limited optical Doppler orbitography measurements of range rate. The unstabilised technique determines the Doppler shift directly from a heterodyne measurement of the returned optical signal. The stabilised technique aims to improve the precision of the first by suppressing atmospheric phase noise imprinted on the transmitted optical signal. We demonstrate the performance of each technique over a $$2.2\,\hbox {km}$$ horizontal link with a simulated in-line velocity Doppler shift at the far end. A horizontal link of this length has been estimated to exhibit nearly half the total integrated atmospheric turbulence of a vertical link to space. Without stabilisation of the atmospheric effects, we obtained an estimated range rate precision of $$17\,{\upmu }\hbox {m}\,\hbox {s}^{-1}$$ at $$1\,\hbox {s}$$ of integration. With active suppression of atmospheric phase noise, this is improved by three orders of magnitude to an estimated range rate precision of $$9.0\,\hbox {nm}\,\hbox {s}^{-1}$$ at $$1\,\hbox {s}$$ of integration, and $$1.1\,\hbox {nm}\,\hbox {s}^{-1}$$ when integrated over $$60\,\hbox {s}$$ . This represents four orders of magnitude improvement over the typical performance of operational ground to space X-Band systems in terms of range rate precision at the same integration time. The performance of this system is a promising proof of concept for coherent optical Doppler orbitography. There are many additional challenges associated with performing these techniques from ground to space that were not captured within the preliminary experiments presented here. In the future, we aim to progress towards a $$10\,\hbox {km}$$ horizontal link to replicate the expected atmospheric turbulence for a ground to space link.

Free-space transfer of comb-rooted optical frequencies over an 18\u2009km open-air link

Phase-coherent transfer of optical frequencies over a long distance is required for diverse photonic applications, including optical clock dissemination and physical constants measurement. Several demonstrations were made successfully over fiber networks, but not much work has been done yet through the open air where atmospheric turbulence prevails. Here, we use an 18 km outdoor link to transmit multiple optical carriers extracted directly from a frequency comb of a 4.2 THz spectral width. In stabilization to a high-finesse cavity with a 1.5 Hz linewidth, the comb-rooted optical carriers are simultaneously transferred with collective suppression of atmospheric phase noise to −80 dBc Hz−1. Microwaves are also delivered by pairing two separate optical carriers bound with inter-comb-mode coherence, for example a 10 GHz signal with phase noise of −105 dBc Hz−1 at 1 Hz offset. Lastly, an add-on demonstration is given for multi-channel coherent optical communications with the potential of multi-Tbps data transmission in free space.

WAYS OF EXTRACTION OF MAXIMUM INFORMATION FROM IMAGES OF ASTRONOMICAL OBJECTS

Generally, the information obtained as a result of a physical experiment proves to be partially corrupted due to action of various noise factors. The nature of these factors is dependent on the experimental conditions however, approaches to reducing their influence prove to be rather similar. The investigations carried out by the team of image processing of the Institute of Radio Physics and Electronics, National Academy of Sciences of Ukraine (IRE NASU) over the last ten years were oriented toward search for methods of extraction of maximum information from astronomical observations and space experiments. The investigations were conducted in several directions. For observations from the Earth's surface, the problem consisted in removing atmospheric phase distortions and synthesizing antenna configurations for telescopes and interferometers operating in various ranges of the electromagnetic wave spectrum. In the case of space astronomy, the efforts were concentrated at optimal interpretation of their results, in particular, at obtaining the planet relief and image processing with the effect of gravitational lensing. The common feature of the developed techniques is the use of the statistical approach, i.e., application of the mathematical statistics and theory of optimal statistical solutions. The present paper discloses both the general features of its approach and specific features of the obtained solutions in the case of particular problems of astronomical image processing. As a result of the performed investigation several techniques have been developed, including the method of Fourier component total phase accumulation for suppression of the atmospheric phase noise, the technique of optimum estimation of the point source intensity observed through the gravitational field of a distant galaxy, the method of accounting the altimetric information in photoclinometric determining the planet surface relief, and a number of techniques of designing antenna configurations for telescopes and interferometers. The statistical approach to processing experimental data has allowed using the information contained in a series of observations of an astronomical object for reconstruction of a single but more accurate its image. Owing to this it has become possible to unite in an optimal way the information contained in the surface images and altimetric data to obtain the surface relief with a high resolution and referencing to actual altitude magnitudes. In the case of treating gravitational mirages it has become possible to separate the observed source from the background. The obtained results demonstrate that the Bayesian statistical approach is a powerful tool in the researcher hands.

ATMOSPHERIC PHASE NOISE AND APERTURE SYNTHESIS IMAGING AT MILLIMETER WAVELENGTHS

ABSTRACT We analyze data obtained with the Hat Creek interferometer at 86 GHz to estimate the atmospheric phase noise on spatial scales of 6m to 846m, and time scales of 1 second to 10 hours. We find a phase structure function with a power law index, Beta between 0.6 and 1.7. A slope close to the Kolmogorov 2D value 0.67 is obtained for longer baselines and stable weather conditions; the slope approaches the Kolmogorov 3D value 1.67 only on short baselines and in turbulent weather. The rms phase at 1 km baseline is 1mm, with a variation by a factor 4 over a few days even during good weather conditions. The mean elevation dependence is sinelevation-0.7 ± -0.2. The variation of Beta and the elevation dependence with baseline length are consistent with turbulent regions 100-300 m thick. The resolution and dynamic range of millimeter wavelength aperture synthesis images is limited by atmospheric turbulence. Observing techniques and data analysis procedures to correct for atmospheric seeing are discussed in some detail.

Interferometer measurements of atmospheric phase noise at 86 GHz

We report measurements of the atmospheric phase noise at 86 GHz, made by using the Hat Creek millimeter wavelength interferometer. The measurements used a dual‐frequency technique to estimate the receiver noise contribution to the phase. The data, which cover projected baselines between 2000 and 40000 wavelengths, are used to derive median root mean squared (rms) phase noise as a function of baseline length. The implications of these results for the resolution limit to millimeter wavelength interferometry are discussed. We also discuss the time domain spectrum of atmospheric phase noise and consider an observing strategy that can mitigate the effects of adverse atmospheric conditions.

8.4 - A 3.39-micron infrared optical heterodyne communication system

An optical heterodyne communication system is described which employs a separate stable laser local oscillator at the receiver. The theoretical advantage of quantum-limited reception has been realized, demonstrating an improvement in receiver sensitivity of more than 40 dB over that of a conventional photodetector receiver. The fundamental sources of noise in the system are identified as laser oscillator frequency noise, atmospheric phase noise, atmospheric amplitude noise, and quantum noise. The quantitative characteristics of these noise sources are analyzed as they influence the operation of AM and FM laser communications.