Active Hydrogen Maser Research Articles

Steering of national time scale UTC(KZ) based on passive hydrogen masers using Rapid UTC data

An important characteristic of a national time scale is its deviation from the Coordinated Universal Time (UTC) scale. The emergence of the Rapid Coordinated Universal Time scale (UTCr) offers the potential to form national time scales that closely follow UTC with high accuracy. Information on the deviations of national time scales in Rapid UTC is updated weekly. In this case, it becomes possible to use more cost-effective atomic clocks with greater frequency instability compared to the active hydrogen masers currently employed in leading timekeeping metrology laboratories. The feasibility of using the Rapid Coordinated Universal Time scale (UTCr) to implement a national time scale based on passive hydrogen masers with high UTC tracking accuracy is demonstrated using the example of the national Coordinated Universal Time scale of Kazakhstan, UTC(KZ). The results of the automatic steering system for the UTC(KZ) time scale based on the Rapid UTC data, published by the International Bureau of Weights and Measures, are presented. The steering algorithm is described, and its parameters are justifi ed. It is shown that a time scale based on passive hydrogen masers, with weekly corrections from Rapid UTC data, achieves a root mean square error (RMSE) of 2.5 ns and a maximum deviation of less than 8 ns over a two-year observation period.

The engineering face-lift of the 32m dish: technical upgrades of Ghana Radio Astronomy Observatory (GRAO)

Ghana's 32-meter radio telescope, inaugurated in August 2017, was once a redundant telecommunications dish that underwent conversion. Prior to this transformation, feasibility studies were conducted to assess the dish's structural integrity, technical compatibility, and economic viability. These studies aimed to determine if the conversion project could be technically achieved given the available technology, expertise, and resources. This paper delves into the engineering considerations surrounding structural, mechanical, software, control and monitoring, radio frequency, and timing frequency reference requirements that distinguish the operation of a radio telescope from its former role as a satellite earth station. Significant components such as the azimuth bearing, sub-reflector support, cable wrap, and electrical motors underwent replacement. Additionally, a new C-band receiver, radio frequency controller, active hydrogen maser timing frequency, and software were developed. Testing protocols to meet science requirements for both single-dish observations and Very Long Baseline Interferometry (VLBI) are also discussed. The conversion process proved to be lengthy and encountered numerous unforeseen circumstances, yet it provided invaluable learning experiences for a developing country like Ghana.

Optical clocks at sea

Deployed optical clocks will improve positioning for navigational autonomy1, provide remote time standards for geophysical monitoring2 and distributed coherent sensing3, allow time synchronization of remote quantum networks4,5 and provide operational redundancy for national time standards. Although laboratory optical clocks now reach fractional inaccuracies below 10−18 (refs. 6,7), transportable versions of these high-performing clocks8,9 have limited utility because of their size, environmental sensitivity and cost10. Here we report the development of optical clocks with the requisite combination of size, performance and environmental insensitivity for operation on mobile platforms. The 35 l clock combines a molecular iodine spectrometer, fibre frequency comb and control electronics. Three of these clocks operated continuously aboard a naval ship in the Pacific Ocean for 20 days while accruing timing errors below 300 ps per day. The clocks have comparable performance to active hydrogen masers in one-tenth the volume. Operating high-performance clocks at sea has been historically challenging and continues to be critical for navigation. This demonstration marks a significant technological advancement that heralds the arrival of future optical timekeeping networks.

Research on an Active Hydrogen Maser Digital Circuit Control System Based on FPGA.

A hydrogen maser is a high-precision time measurement instrument with high frequency stability and low frequency drift, which is widely used in satellite navigation, ground time keeping, frequency measurement, and other fields. An active hydrogen maser (AHM) is better than the current space passive hydrogen maser (PHM) in orbit in terms of its frequency stability and drift rate, but it has the disadvantages of large volume and weight. To further reduce the volume and weight of the circuit, this paper demonstrates a digital circuit control system based on a field-programmable gate array (FPGA). It uses digital temperature control, digital detectors, digital down-conversion, digital phase-locked loops, and other digital methods for temperature control, cavity auto-tuning, and crystal phase locking, which improve the integration and flexibility of the circuit system. Meanwhile, a tuning method based on hydrogen flow is proposed, which effectively solves the problem of fluctuations in hydrogen maser resonance frequency with changes in the external environment. Our experimental results show that the designed digital circuit control system meets the requirements of an oven-controlled crystal oscillator (OCXO) loop and a cavity loop. Its frequency stability can reach 2.6×10-13/1 s and 1.4×10-15/10,000 s, which is close to the stability index of ground active hydrogen maser. This scheme has certain practical engineering value, and can be used in the design of hydrogen masers for next-generation space navigation satellites, deep space exploration, and space stations.

Multi-nodes dissemination of stable radio frequency with 10-17 instability over 2000 km optical fiber.

To meet the demand of flexible access for high-precision synchronization frequency, we demonstrate multi-node stable radio frequency (RF) dissemination over a long-distance optical fiber. Stable radio frequency signals can be extracted at any node along the optical fiber, not just at the endpoint. The differential mixing structure (DMS) is employed to avoid the frequency harmonic leakage and enhance the precision. The phase-locked loop (PLL) provides frequency reference for the DMS while improving the signal to noise ratio (SNR) of dissemination signal. We measure the frequency instability of multi-node stable frequency dissemination system (MFDS) at different locations along the 2,000 km optical fiber. The measured short-term instability with average time of 1 s are 1.90 × 10-14 @ 500 km, 2.81 × 10-14 @ 1,000 km, 3.46 × 10-14 @ 1,500 km, and 3.84 × 10-14 @ 2,000 km respectively. The long-term instability with average time of 10,000 s are basically the same at any position of the optical fiber, which is about (6.24 ± 0.05) × 10-17. The resulting instability is sufficient for the propagation of precision active hydrogen masers.

Characteristics of a real-time ensemble time scale corrected by the KRISS-PSFS using a reduced Kalman filter (Kred) algorithm

An ensemble time, which is indicated by TA (temps atomique, in French), was generated every minute by combining four active hydrogen masers with the reduced Kalman filter (Kred) algorithm. The TA frequency was averaged over 12 h and then the mean frequency was steered to the primary and secondary frequency standards developed by KRISS with a micro-phase stepper (MPS). The steered and physically realized TA frequency is called KRISS ensemble time (KET). We investigated the frequency-state estimate outputted from the Kred algorithm and the control signal applied to the MPS, according to the abnormal temperature change of the room where the hydrogen masers were placed. When the frequency of the TA was corrected by the KRISS-F1, the Allan deviation of KET was 1.72 × 10−15 at the average time τ = 1 d, and the variation of [UTC–KET] was stable within 2 ns for 75 d.

Recent progress on quantum frequency standards at BIRMM

Quantum frequency standards are crucial for time measurement, satellite navigation, telecommunication, and other essential applications. Beijing Institute of Radio Metrology and Measurement (BIRMM) has been working on quantum frequency standards and their applications for tens of years. This paper introduces the latest progress on quantum frequency standards at BIRMM, including a calcium optical clock, an active hydrogen maser, and a mercury ion microwave clock. Based on the 1S0-3P1 transition of calcium atoms, a transportable optical clock prototype is built with a stability of 8 × 10–15 at 1 s. A compact active hydrogen maser has been developed for the Chinese space station. It will be used for scientific research such as examining Einstein’s theory of general relativity and has just been lunched. The preliminary frequency stability of the maser is 1.27 × 10–15 at 10000 s. Additionally, a prototype mercury ion microwave clock is developed using the hyperfine transition between 62S1/2, F = 0 and 62S1/2, F = 1. The trapped Hg+ ions are pumped by mercury discharge lamps and cooled by Helium gas. The measured clock transition linewidth is about 1 Hz, and frequency stability of 4 × 10–13 at 1 s and 4 × 10–14 at 1000 s is achieved.

Design of a vacuum system for space active hydrogen maser

With the high precision and stability of its frequency signal outputs, active hydrogen maser plays an important role in such fields as timing, satellite navigation, and communication. However, it needs to be lighter so as to be applied in space. We made a research, based on the calculation of the hydrogen flow and the adsorption efficiency of the adsorption unit, on the parameters of the vacuum system and the structural requirements, and designed a combined vacuum pump for the Space Active Hydrogen Maser (SAHM). This vacuum pump consists of a getter pump and a small ion pump, the total mass of which is about 5 kg. The pumping speed will be about 474 L/s by computation, when an amount, 2.5 MPa L, of hydrogen has been adsorbed by getters. Theoretically, the total source hydrogen inflow in lifetime is not higher than 20% of the total capacity getter pump, thus the design should amply meet the requirements of the SAHM vacuum system, and is of great significance for future SAHM applications.

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.

Improvement of the frequency stability of the cesium fountain clock by data processing

The cesium fountain clock is used as the primary frequency standard to improve the accuracy of the excitation source (active hydrogen maser) through cesium atomic transition frequency. The authors find that the frequency stability of cesium fountain clock is poorer than the active hydrogen maser according to the laws of the frequency stability evaluated by the Allan variance. The cesium fountain frequency stability can only reach 2 × 10−15 after 10 000 s. The traditional stability improvement methods are based on the Ramsey line-width optimization. The correction signal of fountain clock contains error, which can be removed partly by data correction. A new method to improve the frequency stability by modified error factor with statistical theory is presented in this paper. The results of frequency stability can reach 1.03 × 10−14/2 s and 2.78 × 10−16/16 400 s.

Optimal design and validation of atom trapping and atomic storage time for active hydrogen maser.

From microwave atomic clocks to light clocks, atomic or ionic clocks often rely on atom or ion trapping or manipulation technology. Trapping hydrogen (H) atoms in atomic storage bulbs (ASBs) is one of the key technologies of H atomic clocks. H atoms remain in an ASB for some time during which they undergo several relaxation processes (including spin-exchange collision relaxation, atom-wall collision relaxation, and magnetic-field inhomogeneity relaxation) and interact with the electromagnetic field within the resonant cavity in the TE011 mode, giving rise to continuous atomic transitions and self-oscillations. In this study, an optimal atomic storage time Tb for a H maser was determined by optimizing various collisional relaxation times of the atomic ensemble and reducing the width of the atomic resonance line through the continuously adjustable length and radius of the opening of an ASB at various atomic beam intensities ξ (which is the number of atoms in the atomic beam), namely, 3 × 1012 atoms/s, 4 × 1012 atoms/s, and 5 × 1012 atoms/s, while keeping the structural properties and physical conditions of the H maser unchanged. For ξ = 5 × 1012 atoms/s and Tb ≈ 0.8s, a frequency stability of 0.95 × 10-15 could be achieved at 1000s.

Detecting Cycle Slips in Carrier-Phase Measurements of Single Frequency Navigation Receivers with Different Instabilities of Reference Oscillators

The use of carrier-phase measurements significantly increases the accuracy of solutions when using the measurements of navigation receivers. One of the problems in carrier-phase measurements is discontinuities (cycle slips) in the measurements. The existing algorithms of detection and compensation of cycle slips in carrier-phase measurements of a singlefrequency navigation receiver either require additional information (for example, Doppler measurements), or operate only in differential mode, or can only detect large cycle slips. The purpose of the research is the development of algorithms for detecting small cycle slips in carrier-phase measurements of single-frequency receivers without using additional information. We use methods of filtering of the trend in the carrier-phase measurements using polynomial or adaptive bases, as well as modified sparse recovery algorithms to estimate cycle slips in the difference between code and carrier-phase measurements. The algorithm which is used to search cycle slips in carrier-phase measurements depends on the quality of the reference oscillator of the navigation receiver. For receivers with high-stability reference oscillators (e.g. active hydrogen maser), one can use polynomial filtering of the trend, the filtering result directly detects discontinuities in carrier-phase measurements with a probability close to unity. For navigation receivers with low-stability reference oscillators (quartz reference oscillators), a modified algorithm for minimization of the total variation with filtering of the trend applied to the difference between the code and carrier-phase single-frequency measurements detects discontinuities in 1 cycle slip against the background of the noise component of comparable magnitude with a probability of 0.8. The results may be applied in navigation systems with single-frequency receivers with low stability reference oscillators, as well as in a posteriori processing of receivers’ measurements to correct carrier-phase measurements on the preprocessing stage.

Realization of a free-running timescale based on a CsF1 cesium fountain clock and multiple active hydrogen masers

It is important to realize a stand-alone timescale with high-quality stability in short- and long-terms that can function accurately without any external inputs. A free-running timescale (FreeAT) was calculated as a weighted average of five active hydrogen masers and has operated freely for more than 400 d in the Beijing Satellite Navigation Center (BSNC). In the weighted average algorithm, a cesium fountain clock (CsF1) with a typical Type-A uncertainty of 4.50 × 10−16 for averaging times of 30 d was used as the frequency reference to estimate the frequency drifts of hydrogen masers. Additionally, the time difference between the rapid realization of Coordinated Universal Time (UTCr) and FreeAT was obtained indirectly by utilizing the timescale signal comparison between the BSNC and National Institute of Metrology via an optical fiber link. The overlapping Allan deviation of FreeAT with respect to UTCr is 8.69 × 10−16 for averaging times of 15 d, and its frequency difference and drift relative to UTCr are −1.20 × 10−16 and −2.08E-18/day, respectively. Therefore, the combination of a fountain and maser ensemble can generate a high-performance time scale.

Long-term digital frequency-stabilized laser source for large-scale passive laser gyroscopes.

We report on the development of a digitally controlled long-term frequency stabilized ultrastable laser source, which serves as an injection laser to stabilize the perimeter of a 3 m × 3 m heterolithic passive resonant gyroscope. We operate the gyroscope at two different cavity modes to reduce back-scattering coupling disturbance for gyroscope locking. This scheme increases the requirement for the injection laser frequency stability since we are using the wavelength of the laser as the length standard for the heterolithic gyroscope structure. The laser source is digitally locked to an ultrastable high-finesse Fabry-Perot cavity and a femtosecond optical frequency comb referenced to an active hydrogen maser simultaneously. The fractional frequency stability of the locked laser is better than 1.2 × 10-14 for averaging times from 0.1 s to 10 000 s. The short-term frequency stability is limited by the stability of the Fabry-Perot cavity, and the long-term frequency stability is limited by the stability of the frequency comb. The digital locking system enables the laser to run autonomously for weeks and can quickly relock itself within seconds to ensure continuous running of the gyroscope. The digital frequency stabilization technique can also fulfill the requirements of space gravitational waves detection and the next generation space gravity recovery mission.

Atomic Clock Performance Assessment of BeiDou-3 Basic System with the Noise Analysis of Orbit Determination and Time Synchronization

The basic system of the BeiDou global navigation satellite system (BDS-3) with 18 satellites has been deployed since December 2018. As the primary frequency standard, BDS-3 satellites include two clock types with the passive hydrogen maser (PHM) and the rubidium atomic frequency standard (RAFS). Based on the final precise orbit and clock product from Xi’an Research Institute of Surveying and Mapping (XRS), the atomic clock performance of BDS-3 satellites is evaluated, including the frequency accuracy, frequency drift rate, and frequency stability, and compared with GPS block IIF satellites with RAFS, Galileo satellites with PHM, and BDS-2 satellites. A data auto-editing procedure to preprocess clock data and assess the clock performance is developed, where the assessed results are derived at each continuous data arc and the outliers are excluded properly. The stability of XRS product noise is given by using some stations equipped with high-precision active hydrogen masers (AHM). The best stability is 8.93 × 10−15 and 1.85 × 10−15 for the averaging time of 10,000 s and 1 day, which is basically comparable to one-third of the in-orbit PHM frequency stability. The assessed results show the average frequency accuracy and drift rate of BDS-3 with RAFS are slightly worse while the stability is better than BDS-2 medium earth orbit (MEO) satellites. The 10,000 s stability is better but the 1-day stability is worse than GPS, which may be related to the performance of the BDS-3 RAFS clock. As for BDS-3 with PHM, the frequency accuracy is slightly worse than Galileo PHM satellites; the drift rate, when excluding C34 and C35, is basically comparable to Galileo and significantly better than GPS satellites; the stability is comparable to Galileo, where the 10,000 s stability is slightly worse than Galileo and better than GPS. The 1-day stability among BDS-3 PHM, GPS IIF RAFS, and Galileo PHM satellites is basically comparable.

Research on the Methods of Performance Monitoring and Evaluation for Active Hydrogen Maserstwo

Active hydrogen maser is the main frequency standard for establishing and maintaining the time scale. It has the characteristics of high short-term stability and low phase noise. At present, it plays an increasingly important role in the international atomic time (TAI) and various local time scales. Firstly, in combination with the internal state parameters of an active hydrogen maser, the correlation between the internal state parameters and the comparison data of a hydrogen maser is analyzed, and a method for the performance monitoring of a hydrogen maser is proposed. Secondly, according to the characteristics of hydrogen maser performance, a method for evaluating the performance of a hydrogen maser is given. The hydrogen maser performance includes mainly two aspects, namely the frequency stability and predictability. The performances of two types of active hydrogen masers (CH1-75 and MHM-2010) are evaluated by this method. The correlation analysis of the atomic clock state parameters and comparison data shows that the state parameter monitoring can effectively predict the variation of the hydrogen maser performance. The evaluation results of atomic clock frequency stability and predictability show that the atomic clock with a higher medium-and-long term frequency stability has a better predictability. There are two methods for the predictability evaluation, one is based on the data published by BIPM (Bureau International des Poids et Measures), and another one is based on the quadratic model. Both methods are consistent with the weights published by BIPM. Therefore, the two methods can be used as a quantitative method to evaluate the predictability of a hydrogen maser.

Spectroscopy of the S01−D21 clock transition in Lu+176

High precision spectroscopy of the $^1S_0$-to-${^1}D_2$ clock transition of $^{176}$Lu is reported. Measurements are performed with Hertz level precision with the accuracy of the hyperfine-averaged frequency limited by the calibration of an active hydrogen maser to the SI definition of the second via a GPS link. The measurements also provide accurate determination of the $^1D_2$ hyperfine structure. Hyperfine structure constants associated with the magnetic octupole and electric hexadecapole moments of the nucleus are considered, which includes a derivation of correction terms from third-order perturbation theory.

A Modem with a Fiber-Optic Communication Line for Dissemination of Frequency and Time Reference Signals

The features of a multichannel modem that has been developed for transmission of frequency and time reference signals on a fiber-optic communication line with phase instability compensation are examined. Its capabilities for transmitting signals of reference frequencies of active hydrogen masers (such as the Ch1-1003M), we well as a 1 PPS signal with an error of synchronization no greater than 250 psec over 100 km without the use of optical amplifiers are assessed. The results of experimental verification of the metrological characteristics of the modem are given.

State selection system main features for active hydrogen maser

State selection system main features for active hydrogen maser

A tunable low-drift laser stabilized to an atomic reference

We present a laser system with a linewidth and long-term frequency stability at the 50 kHz level. It is based on a Ti:Sapphire laser emitting radiation at 882 nm which is referenced to an atomic transition. For this, the length of an evacuated transfer cavity is stabilized to a reference laser at 780 nm locked to the $^{85}$Rb D$_2$-line via modulation transfer spectroscopy. Gapless frequency tuning of the spectroscopy laser is realized using the sideband locking technique to the transfer cavity. In this configuration, the linewidth of the spectroscopy laser is derived from the transfer cavity, while the long-term stability is derived from the atomic resonance. Using an optical frequency comb, the frequency stability and linewidth of both lasers are characterized by comparison against an active hydrogen maser frequency standard and an ultra-narrow linewidth laser, respectively. The laser system presented here will be used for spectroscopy of the $1s^{2}2s^{2}2p\ ^{2}P_{1/2} -\ ^{2}P_{3/2}$ transition in sympathetically cooled Ar$^{13+}$ ions at 441nm after frequency doubling.