Optical Reference Research Articles

High-Precision Time-Frequency Signal Simultaneous Transfer System via a WDM-Based Fiber Link

In this paper, we demonstrate a wavelength division multiplexing (WDM)-based system for simultaneously delivering ultra-stable optical frequency reference, 10 GHz microwave frequency reference, and a one pulse per second (1 PPS) time signal via a 50 km fiber network. For each signal, a unique noise cancellation technique is used to maintain their precision. After being compensated, the transfer frequency instability in terms of the overlapping Allan deviation (OADEV) for the optical frequency achieves 2 × 10−17/s and scales down to 2 × 10−20/10,000 s, which for the 10 GHz microwave reference, approaches 4 × 10−15/s and decreases to 1.4 × 10−17/10,000 s, and the time uncertainty of the 1 PPS time signal along the system is 2.08 ps. In this scheme, specific channels of WDM are, respectively, occupied for different signals to avoid the possible crosstalk interference effect between the transmitted reference signals. To estimate the performance of the above scheme, which is also demonstrated in this 50 km link independent of these signals, the results are similar to that in the case of simultaneous delivery. This work shows that the WDM-based system is a promising method for building a nationwide time and frequency fiber transfer system with a communication optical network.

Double Rayleigh scattering in a digitally enhanced, all-fiber optical frequency reference.

We demonstrate a passive, all-optical fiber frequency reference using a digitally enhanced homodyne interferometric phase readout. We model the noise contributions from fiber thermal noise and the coupling of double Rayleigh scattering in a digitally enhanced homodyne interferometer. A system frequency stability of 0.1 Hz/Hz is achieved above 100 Hz, which coincides with the double Rayleigh scattering estimate and is approximately a factor of five above the thermo-dynamic noise limit.

Assessment of Low-Cost Particulate Matter Sensor Systems against Optical and Gravimetric Methods in a Field Co-Location in Norway

The increased availability of commercially-available low-cost air quality sensors combined with increased interest in their use by citizen scientists, community groups, and professionals is resulting in rapid adoption, despite data quality concerns. We have characterized three out-the-box PM sensor systems under different environmental conditions, using field colocation against reference equipment. The sensor systems integrate Plantower 5003, Sensirion SPS30 and Alphasense OCP-N3 PM sensors. The first two use photometry as a measuring technique, while the third one is an optical particle counter. For the performance evaluation, we co-located 3 units of each manufacturer and compared the results against optical (FIDAS) and gravimetric (KFG) methods for a period of 7 weeks (28 August to 19 October 2020). During the period from 2nd and 5th October, unusually high PM concentrations were observed due to a long-range transport episode. The results show that the highest correlations between the sensor systems and the optical reference are observed for PM1, with coefficients of determination above 0.9, followed by PM2.5. All the sensor units struggle to correctly measure PM10, and the coefficients of determination vary between 0.45 and 0.64. This behavior is also corroborated when using the gravimetric method, where correlations are significantly higher for PM2.5 than for PM10, especially for the sensor systems based on photometry. During the long range transport event the performance of the photometric sensors was heavily affected, and PM10 was largely underestimated. The sensor systems evaluated in this study had good agreement with the reference instrumentation for PM1 and PM2.5; however, they struggled to correctly measure PM10. The sensors also showed a decrease in accuracy when the ambient size distribution was different from the one for which the manufacturer had calibrated the sensor, and during weather conditions with high relative humidity. When interpreting and communicating air quality data measured using low-cost sensor systems, it is important to consider such limitations in order not to risk misinterpretation of the resulting data.

High stability laser locking to an optical cavity using tilt locking.

This paper describes, to our knowledge, the first demonstration of high performance tilt locking, a method of stabilizing laser frequency to an optical reference cavity using a spatial-mode readout technique. The experiment utilized a traveling wave cavity with a finesse of approximately 10,000, housed in a thermally controlled vacuum chamber. The tilt locking method in a double pass configuration has promising performance in the 100 µHz-1 Hz band, including surpassing the Gravity Recovery and Climate Experiment (GRACE) Follow-On laser ranging interferometer requirement. Tilt locking offers a number of benefits such as high sensitivity, low cost, and simple implementation and therefore should be considered for future applications requiring high performance laser locking, such as future laser-based satellite geodesy missions and the Laser Interferometer Space Antenna.

On the use of luminescent single crystals as optical reference materials

Luminescent single crystals are widely used as laser gain media, scintillators, and recently even as radiation converters in some inorganic LEDs. The present work concerns the use of single crystalline Y3Al5O12:Ce3+ as a reference material for photoluminescence spectroscopy and quantum efficiency determination. Garnet crystals are cubic and thus show isotropic optical properties, which is in contrast to monoclinic materials, such as KEu(WO4)2 with anisotropic optical properties. They are also known for their high stability and thus are in line with the aim to use a single crystalline material with consistent optical properties, both for everyday routine control of a given spectrometer setup and for better comparability between different spectrometer setups. Garnet crystals are particularly stable over a long period and rather easy to handle, with implications for improved reproducibility and repeatability of optical measurements. The advantages and disadvantages of this approach as well as the requirements on the quality and preparation of the Y3Al5O12:Ce3+ crystalline samples are presented. Application examples are shown as well, together with comparisons with conventional reference materials based on powders and solutions.

Optical frequency standard with a single 171Yb+ ion

We report recent results on the development of an optical frequency standard based on the quadrupole transition in a single 171Yb+ ion. The probe laser frequency is simultaneously stabilised to the transmission peak of a Fabry – Perot reference cavity and the central resonance of the quadrupole transition of the ion with a linewidth of 30 Hz. With the experimentally obtained spectral characteristics of the probe laser and optical reference, a daily frequency stability on the order of 10−17 is demonstrated for the 171Yb+ standard.

Functional Principal Component Analysis of Radio–Optical Reference Frame Tie

Abstract The Gaia optical reference frame is intrinsically undefined with respect to global orientation and spin, so it needs to be anchored in the radio-based International Celestial Reference Frame (ICRF) to provide a referenced and quasi-inertial celestial coordinate system. The link between the two fundamental frames is realized through two samples of distant extragalactic sources, mostly active galactic nuclei and quasars, but only the smaller sample of radio-loud ICRF sources with optical counterparts is available to determine the mutual orientation. The robustness of this link can be mathematically formulated in the framework of functional principal component analysis using a set of vector spherical harmonics to represent the differences in celestial positions of the common objects. The weakest eigenvectors are computed, which describe the greatest deficiency of the link. The deficient or poorly determined terms are specific vector fields on the sphere that carry the largest errors of absolute astrometry using Gaia in reference to the ICRF. This analysis provides guidelines for the future development of the ICRF maximizing the accuracy of the link over the entire celestial sphere. A measure of the robustness of a least-squares solution, which can be applied to any linear model fitting problem, is introduced to help discriminate between reference frame tie models of different degrees.

Optical Multiplexing of Metrological Time and Frequency Signals in a Single 100-GHz-Grid Optical Channel.

In this article, the concept of co-locating all metrological time and frequency signals in a single optical channel of a standard, 100-GHz-spaced optical grid is presented and evaluated. The solution is intended for situations where only a narrow optical bandwidth is available in a fiber heavily loaded with standard data traffic. We localized the optical reference signals in the middle of the channel, with signals related to RF reference and time tags shifted ±12.5 GHz apart. In the experimental evaluation with a 260-km-long fiber, we demonstrate that the stability of frequency signals and the calibration of time tags remained at the very same level of stability and accuracy as for systems utilizing separate channels: the fractional long-term instability for the optical frequency reference was below 5 ×10-20 , that for the RF reference at the level of 10-17, and the mismatch of the time tag calibration was not more than 10 ps. We also identify possible issues, mainly related to a risk of unwanted Brillouin amplification and scattering.

Prototype of a compact rubidium-based optical frequency reference for operation on nanosatellites

Space-borne optical frequency references based on spectroscopy of atomic vapors may serve as an integral part of compact optical atomic clocks, which can advance global navigation systems or can be utilized for earth observation missions as part of laser systems for cold atom gradiometers. Nanosatellites offer low launch costs, multiple deployment opportunities, and short payload development cycles, enabling rapid maturation of optical frequency references and underlying key technologies in space. Toward an in-orbit demonstration on such a platform, we have developed a CubeSat-compatible prototype of an optical frequency reference based on the D2 transition in rubidium. A frequency instability of 1.7 × 10 − 12 at 1 s averaging time is achieved. The optical module occupies a volume of 35 c m 3 , weighs 73 g, and consumes 780 mW of power.

Development of a system for monitoring performance in the strength and conditioning domain

The development hardware and software aspects of a novel system for monitoring performance in the strength and conditioning domain are summarized in this paper. A development framework is presented to enable the capture of development processes and support detailed comparison of performance monitoring systems. The framework couples the established Computer Integrated Manufacture Open Systems Architecture (CIMOSA) with Unified Modelling Language (UML) to document both the software development and implementation. Corrective algorithms are proposed that have increased the accuracy of the IMU outputs in monitoring the performance of a countermovement jump. Lastly, a wireless Inertial Measurement Unit (IMU) has been validated for use within the strength and conditioning domain by comparison with an optical motion capture reference measure.

Arbitrary Phase Access for Stable Fiber Interferometers

AbstractWell‐controlled yet practical systems that give access to interference effects are critical for established and new functionalities in ultrafast signal processing, quantum photonics, optical coherence characterization, etc. Optical fiber systems constitute a central platform for such technologies. However, harnessing optical interference in a versatile and stable manner remains technologically costly and challenging. Here, degrees of freedom native to optical fibers, i.e., polarization and frequency, are used to demonstrate an easily deployable technique for the retrieval and stabilization of the relative phase in fiber interferometric systems. The scheme gives access (without intricate device isolation) to <1.3 × 10−3 π rad error signal Allan deviation across 1 ms to 1.2 h integration times for all tested phases, ranging from 0 to 2π. More importantly, the phase‐independence of this stability is shown across the full 2π range, granting access to arbitrary phase settings, central for, e.g., performing quantum projection measurements and coherent pulse recombination. Furthermore, the scheme is characterized with attenuated optical reference signals and single‐photon detectors, and extended functionality is demonstrated through the use of pulsed reference signals (allowing time‐multiplexing of both main and reference signals). Finally, the scheme is used to demonstrate radiofrequency‐controlled interference of high‐dimensional time‐bin entangled states.

Frequency microcomb stabilization via dual-microwave control

Optical frequency comb technology has been the cornerstone for scientific breakthroughs in precision metrology. In particular, the unique phase-coherent link between microwave and optical frequencies solves the long-standing puzzle of precision optical frequency synthesis. While the current bulk mode-locked laser frequency comb has had great success in extending the scientific frontier, its use in real-world applications beyond the laboratory setting remains an unsolved challenge due to the relatively large size, weight and power consumption. Recently microresonator-based frequency combs have emerged as a candidate solution with chip-scale implementation and scalability. The wider-system precision control and stabilization approaches for frequency microcombs, however, requires external nonlinear processes and multiple peripherals which constrain their application space. Here we demonstrate an internal phase-stabilized frequency microcomb that does not require nonlinear second-third harmonic generation nor optical external frequency references. We demonstrate that the optical frequency can be stabilized by control of two internally accessible parameters: an intrinsic comb offset ξ and the comb spacing frep. Both parameters are phase-locked to microwave references, with phase noise residuals of 55 and 20 mrad respectively, and the resulting comb-to-comb optical frequency uncertainty is 80 mHz or less. Out-of-loop measurements confirm good coherence and stability across the comb, with measured optical frequency instability of 2 × 10−11 at 20-second gate time. Our measurements are supported by analytical theory including the cavity-induced modulation instability. We further describe an application of our technique in the generation of low noise microwaves and demonstrate noise suppression of the repetition rate below the microwave stabilization limit achieved.

Загадка звездного каталога Коперника (часть третья)

The author of “The Catalog of Zodiac signs and stars” published in De Revolutionibus, was Ptolemy. Copernicus used this catalog and all available observations of the Sun, Moon and planets, for improvement of the theory and tables of their motions. E. P. Fedorov wants to support with the authoritative opinion of Copernicus the idea of replacing the main optic reference frame to radio system JCRF. Having mastered the experience of previous generations Copernicus knew that the principles of establishing coordinate systems do not change along with opinions on the structure of the universe. Had the places of stars in catalogs not been dependent on the location of the Sun, Copernicus would have had no opportunity to prepare the time and coordinate systems required for solution of his problem. In his manuscript he preserved the idea the principles of compiling star catalogs, which were not included in the published edition along with excerpts from the originally conceived chapters. The words irrefutably prove that the idea of establishing the main reference system from VLBJ observations without the place of vernal equinox could not be supported by reference to Copernicus`s opinion.

Optical clock technologies for global navigation satellite systems

Future generations of global navigation satellite systems (GNSSs) can benefit from optical technologies. Especially optical clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies—in combination with optical inter-satellite links—enable new GNSS architectures, e.g., by synchronization of distant optical frequency references within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized setups have been developed, showing frequency instabilities at the 10–15 level for averaging times between 1 s and 10,000 s. We introduce optical clock technologies for applications in future GNSS and present the current status of our developments of iodine-based optical frequency references.

Compact, All-PM Fiber Integrated and Alignment-Free Ultrafast Yb:Fiber NALM Laser With Sub-Femtosecond Timing Jitter

We report a simple and compact design of a dispersion compensated mode-locked Yb:fiber oscillator based on a nonlinear amplifying loop mirror (NALM). The fully polarization maintaining (PM) fiber integrated laser features a chirped fiber Bragg grating (CFBG) for dispersion compensation and a fiber integrated compact non-reciprocal phase bias device, which is alignment-free. The main design parameters were determined by numerically simulating the pulse evolution in the oscillator and by analyzing their impact on the laser performance. Experimentally, we achieved an 88 fs compressed pulse duration with sub-fs timing jitter at 54 MHz repetition rate and 51 mW of output power with 5.5 * 10-5 [20 Hz, 1 MHz] integrated relative intensity noise (RIN). Furthermore, we demonstrate tight phase-locking of the laser's carrier-envelope offset frequency (fceo) to a stable radio frequency (RF) reference and of one frequency comb tooth to a stable optical reference at 291 THz.

Prototype of a monolithic cavity-based ultrastable optical reference for space applications.

We present a compact, monolithic optical reference for the frequency stabilized laser of future inter-satellite laser interferometer missions. A prototype based on the integration of a high-finesse cavity and associate optics has been designed to be space compatible while maintaining sufficient stability. The prototype has then been developed with a space-qualified bonding technique, and an in situ multi-degree-of-freedom alignment method. The performances of the optical reference have been studied by beat note analysis with another frequency stabilized laser, and the preliminary results are in agreement with the potential requirements of future space missions.

Detection of Equine Hoof Motion by Using a Hoof-Mounted Inertial Measurement Unit Sensor in Comparison to Examinations with an Optoelectronic Technique - A Pilot Study

This study aimed to determine whether a hoof-mounted inertial measurement unit (IMU) system would provide similar timing characteristics and step lengths, in walk and trot on different grounds, compared to those of an established optoelectronic motion system. The right hoof of one horse was equipped with an IMU sensor with an optical reference marker on top. Fifteen steps on firm ground, and five on penetrable ground, were performed at walk and trot. Level of agreement between the two measurement systems was determined by assessing the concordance correlation coefficients, accuracy, and precision of the duration of different gait events and step length. Timing characteristics and step length were in strong agreement between the two techniques for the majority of assessed parameters in walk and trot on both grounds. The agreement between techniques decreased for breakover and landing duration at trot on penetrable ground. Disparity between the measurement techniques was related to difficulties in accurately defining single parts of the stance phase with marker-based optoelectronic analysis on penetrable ground. Detailed examinations of different parts of the stance phase are more accurately performed using hoof-mounted IMU sensors. Results emphasise the great potential of IMU sensors for equine motion analysis in daily practice.

Active vibration isolation of ultra-stable optical reference cavity of space optical clock

Space optical clocks with frequency instability better than 1×10−15 are increasingly demanded for PNT (positioning, navigation, and timing) and space physical science. Inevitably, the micro-vibration level in the order of micro-g (μg) is required for ultra-stable optical reference cavity of space optical clocks. The realization of such a strict micro-vibration requirement over broadband frequencies is still technically challenging under complex space disturbances in the order of milli-g (mg). This paper thus proposes a multi-degree of freedom (DOF) active vibration isolation approach for the ultra-stable optical reference cavity to satisfy the stringent micro-vibration requirement. Non-contact voice coil actuators are designed as the driving unit of active vibration isolation. The robust vibration isolation controller is presented considering the control uncertainty caused by the voice coil clearance and the low frequency drift of accelerometers. The loop shaping technology is used to avoid the current drift and achieve high accuracy vibration isolation over the designed bandwidth. The active vibration isolation experimental platform is further developed, and the experimental results validate the effectiveness of the designed multi-DOF active vibration isolation scheme.

Nonlinear enhanced-absorption resonances in compact alkali-vapor cells for applications in quantum metrology

We review three laser spectroscopy schemes developed recently for observation of high-contrast enhanced-absorption nonlinear resonances in small alkali-vapor cells. In our experiments, optical transitions within the cesium D1 line are involved and a probe beam transmission is analyzed. The first spectroscopy method is based on the configuration with two-frequency counter propagating beams, which are linearly polarized in orthogonal directions. This configuration provides observation of high-contrast natural-linewidth resonances superimposed on broad Doppler profiles when the laser frequency is scanned. These resonances have good prospects for developing a miniature optical frequency reference. The second scheme involves two-frequency counter propagating beams with equal circular polarizations and provides observation of subnatural-linewidth resonances when the Raman frequency detuning is scanned. We use these resonances for stabilizing the microwave frequency of a local oscillator (≈ 4.6 GHz). Frequency stability of around 6 × l0-12 is achieved at 1-s averaging using a 5-mm length cell. This result makes the technique attractive for developing a miniature frequency standard in thr microwave range. The third configuration exploits single-frequency counter-propagating beams with linear orthogonal polarizations. The ultrahigh-contrast subnatural-linewidth resonances can be observed when the longitudinal magnetic field is scanned around zero. The possible application is discussed of these resonances in vector atomic magnetometry.

FOKUS II—Space flight of a compact and vacuum compatible dual frequency comb system

Frequency combs downconvert absolute optical frequency references and thereby can significantly advance time and frequency precision in satellite-based navigation systems, fundamental science, earth observation, and many other spaceborne applications. We have developed a compact and vacuum compatible double comb system, thereby minimizing volume, mass, and power consumption compared to its precursor. Apart from redundancy aspects, the two combs enable autonomous mode number determination. Here, we report on the space comb system design and experimental results of a zero-gravity parabolic space flight in connection with an iodine-referenced cw laser system.