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The article describes a new interferometric system for measuring the coil displacement in the BIPM Kibble balance. The system has led to a significant reduction in both the type A and type B uncertainty of the Kibble balance measurement. Compared to the previous version, the new instrument doubles the measurement signal thanks to a novel design. The optical and mechanical setup is more compact, more stable, and less sensitive to vibration noise. It is also more easily aligned, particularly for adjusting the interferometer beams vertically. [ABSTRACT FROM AUTHOR]

LiDARs are becoming one of the pillars for the environmental sensing required by advanced driver assistance system (ADAS). Driven by the automotive industry, many new manufacturers are continuously putting new LiDARs on the market, thus increasing their availability and, concomitantly, reducing prices. Accordingly, LiDARs are today finding many new applications also in other fields, such as agriculture and industrial automation. In this article, we describe and discuss a measurement procedure for the analysis and comparison of the performances of LiDARs and report an example of the results obtained from the characterization of one the most widespread LiDARs—the VLP 16 by Velodyne. The proposed measurement procedure and setup have been designed to allow quick and easy installation and analysis of most LiDARs. In particular, they have been designed to allow the straightforward investigation of performances such as the warm-up, stability, and errors in the measured coordinates and parameters such as the spots pattern, waist, and divergence of the laser beams, thus allowing to analyze probably the most relevant performances and parameters for scanning LiDARs. Errors in the measured coordinates have been estimated using an absolute interferometer, whereas the beam parameters have been analyzed using a camera system. As an example, the analysis of the performances of the VLP 16 revealed a warm-up time of about 42 min and errors of few millimeters over a measuring interval of about (2, 21) m. On the other hand, the analysis of the laser beams revealed the vertical and horizontal beam divergences of $\approx 1.2$ and $\approx 3.9$ mrad, respectively. [ABSTRACT FROM AUTHOR]

Presently, ultra-precise optical clocks’ comparisons can be performed easily using the optical frequency combs (OFCs) as a transfer oscillator. In this scheme, different OFC branches are usually used to enable optical clocks’ comparison at different wavelengths. Some branches may involve processes such as optical amplification, second-harmonic generation, self-phase modulation, and propagation through fibers. Such processes can degrade the stability of the comparison if they are uncommon between branches. In this article, a technique is introduced to investigate the applicability of multibranch OFCs to perform ultra-precise optical frequency comparisons. This technique is based on generating a software-transfer beat (STB) between the OFC branches under investigation and the fundamental or the second-harmonic outputs from a frequency-stabilized laser to a hyperfine component from the two-photon transition 5S $_{1/2} \to 5\text {D}_{5/2}$ in rubidium. In addition, a simple compensation scheme is proposed to enhance the relative stability between OFC branches to enable the comparison of the best available optical clocks. [ABSTRACT FROM AUTHOR]

We develop a measurement scheme for the time and frequency offsets, along with their compensation, for a coherent optical receiver. This relies on a modified constant amplitude zero autocorrelation sequence-based method, which is implemented on a field-programmable gate array. Experiments are performed for an optical orthogonal frequency-division multiplexing transmission system. The performance of the proposed scheme is compared to a MATLAB-based implementation of the original scheme. The contributory factors to the error between these two schemes, as well as the implementation complexity, are investigated. Experimental measurements demonstrate that the proposed scheme closely matches the performance of the original scheme while utilizing fewer hardware resources. [ABSTRACT FROM AUTHOR]

We report on a robust distributed Bragg reflector (DBR) laser system for an atomic fountain clock. To confirm the performance of a DBR laser, the short-term stability of an atomic fountain clock, KRISS-F1, is measured with both DBR laser and an extended-cavity diode laser (ECDL) as a master laser in the fountain laser system. The short-term stability of the fountain clock due to the laser noise is determined to be σy(τ = Tc) = 1.4×10−14 when either the ECDL or the DBR laser is used as a master laser. The total stability of the fountain clock with the DBR laser is limited to 3.5× 10−14 at 1 s of averaging time and at the highest atom number in the experiment, which is limited by quantum projection noise. No additional degradation in fountain performance is observed when an ECDL is replaced by the DBR laser. From these results, we conclude that our fountain clock is made much more robust by adopting a DBR laser, which is immune to acoustic and vibrational shocks as well as environmental temperature variations. [ABSTRACT FROM AUTHOR]

In this paper, we report on the implementation and stability analysis of a drift-compensated frequency synthesizer from a cryogenic sapphire oscillator (CSO) designed for a Cs/Rb atomic fountain clock. The synthesizer has two microwave outputs of 7 and 9 GHz for Rb and Cs atom interrogation, respectively. The short-term stability of these microwave signals, measured using an optical frequency comb locked to an ultrastable laser, is better than 5×10−15 at an averaging time of 1 s. We demonstrate that the short-term stability of the synthesizer is lower than the quantum projection noise limit of the Cs fountain clock, KRISS-F1(Cs) by measuring the short-term stability of the fountain with varying trapped atom number. The stability of the atomic fountain at 1 s averaging time reaches 2.5×10−14 at the highest atom number in the experiment when the synthesizer is used as an interrogation oscillator of the fountain. In order to compensate the frequency drift of the CSO, the output frequency of a waveform generator, in the synthesis chain, is ramped linearly. By doing this, the frequency stability of the synthesizer at an average time of one hour reaches a level of 10−16, which is measured with the fountain clock. [ABSTRACT FROM AUTHOR]

A high-precision laser range finder based on highly stable synchronous measuring wavelengths with a large range is proposed in this paper. In this method, the coarse and fine measuring wavelengths are generated by the beat signals of lasers in different frequencies. The lasers are produced by frequency shifting one laser mode with three acousto-optic modulators working at different frequencies. To realize an appropriate coarse and fine measuring wavelengths for the required measurement range and resolution, the driving frequencies of the acousto-optic modulators are controlled by direct digital synthesizer (DDS) in high precision. At the same time, the frequency drift of the laser source and other common errors are reduced by applying the optical heterodyne effect. Consequently, the stability of measuring wavelengths is significantly improved. Furthermore, an improved digital method of phase measurement with same basic clock DDS is proposed to achieve synchronous phase measurement with a little error. Results from the experiments show that the measuring frequency stability is better than 1.75 \times 10^-9 ( $\tau =100$ s), and a measurement with relative uncertainty of 0.63\times 10^{-6} at a distance of 100 m is achieved under laboratory conditions. [ABSTRACT FROM PUBLISHER]

Vapor cell atomic clocks exhibit reduced frequency stability for averaging time between about one hundred and a few thousand seconds. Here we report a study on the impact of the main parameters on the mid-to-long term instability of a buffer-gas vapor cell Cs clock, based on coherent population trapping (CPT). The CPT signal is observed on the Cs $D1$ line transmission, using a double $\Lambda$ scheme and a Ramsey interrogation technique. The effects on the clock frequency of the magnetic field, the cell temperature, and the laser intensities are reported. We show in particular that the laser intensity shift is temperature dependent. Along with the laser intensity ratio and laser polarization properties, this is one of the most important parameters. [ABSTRACT FROM PUBLISHER]

We report on transferring 10^-16-level fractional frequency stability of a “master laser” operated at 1.5 \mu\m to a “slave laser” operated at 698 nm, using a femtosecond fiber comb as transfer oscillator. With the 698-nm laser, the ^1\S0 - {}^{3}\P0 clock transition of ^87\Sr was resolved with a Fourier-limited linewidth of 1.5 Hz (before: 10 Hz). Potential noise sources contributed by the frequency comb are discussed in detail. [ABSTRACT FROM PUBLISHER]

This paper describes a distributed-feedback laser diode system frequency stabilized by simple saturated absorption technique at 895 nm on the Cs D1 line with a linewidth narrower than 1 MHz. This laser source was developed to be used in a compact cesium atomic clock based on coherent population trapping. The frequency stability of the laser system, evaluated by measuring the beat note of two similar laser systems and characterized by Allan deviation, is measured to be lower than 1 × 10-11 for integration times of up to 2000 s, even achieving 4 × 1012 at 200 s. This laser diode system, mounted in a mechanically stable cage system using easy-to-find commercially available components, remains in locked operation for several weeks without any problem. [ABSTRACT FROM AUTHOR]

A major obstacle for optical clocks is the frequency shift due to blackbody radiation (BBR). We discuss how one can tackle this problem in an optical lattice clock, in our case, ^87 \Sr: first, by a measurement of the dc-Stark shift of the clock transition and, second, by interrogating the atoms in a cryogenic environment. Both approaches rely on transporting ultracold atoms over several centimeters within a probe cycle. We evaluate the mechanical movement of the optical lattice and conclude that it is feasible to transport the atoms over 50 mm within 300 ms. With this transport, a dc-Stark shift measurement will allow reducing the contribution of the BBR to fractional uncertainty below 2 \times 10^-17 at room temperature by improving the shift coefficient, known only from atomic-structure calculations up to now. We propose a cryogenic environment at 77 K that will reduce this contribution to a few times 10^-18. [ABSTRACT FROM AUTHOR]

Hyperfine structures of ^127I2 absorption transitions near 561 nm are observed via modulation transfer spectroscopy by a homemade Nd:YAG monolithic nonplanar ring laser with its second harmonic generation (SHG) using a single-pass PPMgO:LN crystal. The observed I2 transitions are assigned according to theoretical calculation results of the I2 B-X transition. Two Nd:YAG lasers are locked to the 15th component of the ^127I2 P(58)22-1 transition, and the measured frequency stability of these lasers is 1 \times 10^-13 for a 1-s averaging time. Frequency shifts with changing iodine cell vapor pressure and pump power are measured by observing the heterodyne beat between two I2-stabilized lasers. [ABSTRACT FROM AUTHOR]

We report on the setup and characterization of a 698-nm master-slave diode laser system to probe the 1So -3Po clock transition of strontium atoms confined in a 1-D optical lattice. A linewidth on the order of about 100 Hz of the laser system has been measured with respect to an ultrastable 657-nm diode laser with 1-Hz linewidth using a femtosecond fiber comb as a transfer oscillator. The laser has been used to measure the magnetically induced 1So -3Po clock transition in 88Sr, where a linewidth of 165 Hz has been observed. The transfer oscillator method provides a virtual beat signal between the two diode lasers that has been used to phase lock the 698-nm laser to the 1-Hz-linewidth laser at 657 nm, transferring its stability to the 698-nm laser system. [ABSTRACT FROM AUTHOR]

An external-cavity laser diode was frequency locked on the molecular absorption line R(3) of hydrogen iodide at 1.541 µm using the wavelength-modulation technique. Stabilization is realized by means of two feedback loops acting on two different frequency ranges. Measurements of the closed-loop error signal provided a relative frequency stability of a few parts in 10-10 in a 50-kHz acquisition bandwidth. [ABSTRACT FROM AUTHOR]

We obtained fline = (351 721 960 559 ± 39) kHz, k = 1, as the absolute frequency of F = 4 → F′ = 5 transition of cesium D2 line by measuring the frequency of an external cavity diode laser (ECDL) locked to the center of the line with frequency modulation transfer technique. The frequency of ECDL was measured by an optical frequency synthesizer based on a 1-GHz femtosecond laser system linked to the hydrogen maser. To increase the S/N ratio of the beat note signal, a distributed Bragg reflector laser was injection locked to one of the optical comb components. [ABSTRACT FROM AUTHOR]

In this paper, a pair of diode lasers at 674 nm, locked to two independent ultralow-expansion high-finesse cavities, have been characterized by monitoring the beat frequency between them. The individual laser linewidth of 1.4 Hz for a 3-s averaging time broadened to 4 Hz at 30 s. The relative frequency stability of the beat at 1 s is 2.5 × 10-15. One of the lasers has been used to interrogate the 88Sr+ ²S1/2-²D5/2 transition at 674 nm, and a transition linewidth of 9 Hz has been observed. [ABSTRACT FROM AUTHOR]

Frequency-modulation spectroscopy of the CH3D molecule is performed to assess a novel optical frequency standard at 1.54-μm wavelength. Numerical integration of the demodulated first derivative signals provides the Voigt absorption profiles for the R(6,3) transition at 1543.78 nm at different gas pressures. Deconvolution with the Gaussian (Doppler) line provides the corresponding Lorentzian (collisional) line as a function of the gas pressure. Measurements of the collisional broadening contribution, at three different gas pressures, provide a collisional broadening coefficient of 37 MHz/kPa for the R(6,3) line. [ABSTRACT FROM AUTHOR]

We are developing a highly stable optical-microwave frequency link system using an optical frequency comb. We consider the stability degradation factors in the frequency link system to be the synthesis, phase locking, and photo detection system of the repetition frequency. We evaluated these factors by undertaking phase noise measurements and we estimated the stability from these results. These estimations were consistent with experimental results. [ABSTRACT FROM AUTHOR]