Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. Here, we report a comparison of two geographically separated lasers, over the longest ever reported metrological optical fibre link network, measuring 2220 km in length, at a state-of-the-art fractional-frequency instability of 7 × 10 −17 for averaging times between 30 s and 200 s. The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre. Based on the characterisation of the noise in the lasers and optical fibre link network over different timescales, we investigate the potential for disseminating ultrastable light to improve the performance of remote optical clocks.

Here we share the relevant data of the manuscript “Comparing ultrastable lasers at 7×10-17 fractional frequency instability through a 2,220 km optical fibre network”. Raw data was acquired using multiple synchronised, dead-time free frequency counters in Lambda-mode [1]. The integration time for each data point was 1 s. The data provided here have been processed to reflect the fractional frequency difference between the ultrastable lasers at NPL and PTB, scaled to 1542 nm. Specifically, \(y=(\nu_{\text{NPL(ULE)}}\frac{777327}{1126090}-\frac{767233}{767235}\nu_{\text{PTB(Si)}})/194.4 \ \text{THz}\) where \(y\) is the value recorded in the data files, \(\nu_{\text{NPL(ULE)}}\) and \(\nu_{\text{PTB(Si)}}\) are the optical frequencies of the ultrastable lasers at NPL (referenced to a ULE cavity) and PTB (referenced to Si cavity), respectively. The numerators and the denominators of the scaling factors correspond to mode numbers of the optical frequency comb at NPL and PTB, respectively. The expression for \(y\) corresponds to the fractional transfer beat [2] between the NPL and PTB ultrastable lasers. The file “833000_s_874000_s_data_for_fig_2.txt” contains the timeseries data used to compute the modified Allan deviation reported in Fig. 2a. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. A linear drift of 40 mHz s-1 has been removed in these data. The file “432000_s_912077_s_data_for_fig_3.txt” contains the timeseries data used in Fig. 3. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. These data have additionally been high pass filtered with a cut off frequency of 1 mHz to decouple the short-term instability of the optical fibre link from the drift of the ultrastable lasers (with a characteristic time >1000 s), as described in the manuscript. The files “222000_s_232000_s_data_for_supp_fig_1.txt”, “270000_s_288000_s_data_for_supp_fig_1.txt”, “754000_s_765000_s_data_for_supp_fig_1.txt”, “832000_s_890000_s_data_for_supp_fig_1.txt”, contain the timeseries data used to compute the modified Allan deviation reported in Supplementary Fig. 1. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. A linear drift of 40 mHz s-1 has been removed in these data. The temporal starting point is displayed in seconds in the title of the files relative to 00:00 UTC of 2019/07/06. References [1] Dawkins, S. T., McFerran, J. J. & Luiten, A. N. Considerations on the Measurement of the Stability of Oscillators with Frequency Counters. IEEE Transactions on ultrasonics, ferroelectrics, and frequency control 54, 918-925 (2007). [2] Telle, H.R., Lipphardt, B. & Stenger, J. Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements. Appl. Phys. B 74, 1-6 (2002).

We search for transient variations of the fine structure constant using data from a European network of fiber-linked optical atomic clocks. By searching for coherent variations in the recorded clock frequency comparisons across the network, we significantly improve the constraints on transient variations of the fine structure constant. For example, we constrain the variation in alpha to <5*10^-17 for transients of duration 10^3 s. This analysis also presents a possibility to search for dark matter, the mysterious substance hypothesised to explain galaxy dynamics and other astrophysical phenomena that is thought to dominate the matter density of the universe. At the current sensitivity level, we find no evidence for dark matter in the form of topological defects (or, more generally, any macroscopic objects), and we thus place constraints on certain potential couplings between the dark matter and standard model particles, substantially improving upon the existing constraints, particularly for large (>~10^4 km) objects.

We present a measurement of the absolute frequency of the 5s 2 1 S 0 to 5s5p 3 P 0 transition in 87 Sr, which is a secondary representation of the SI second.We describe the optical lattice clock apparatus used for the measurement, and we focus in detail on how its systematic frequency shifts are evaluated with a total fractional uncertainty of 1 × 10 -17 .Traceability to the International System of Units is provided via comparison to International Atomic Time (TAI).Gathering data over 5-and 15-day periods, with the lattice clock operating on average 74% of the time, we measure the frequency of the transition to be 429 228 004 229 873.1(5)Hz, which corresponds to a fractional uncertainty of 1 × 10 -15 .We describe in detail how this uncertainty arises from the intermediate steps linking the optical frequency standard, through our local time scale UTC(NPL), to an ensemble of primary and secondary frequency standards which steer TAI.The calculated absolute frequency of the transition is in good agreement with recent measurements carried out in other laboratories around the world.

We developed a table-top multipass spectrograph with a 610-MHz resolution (corresponding to a resolving power of 450,000) and a throughput of 10%. The spectrograph was calibrated with a 4-GHz optical frequency comb (OFC) that did not require filtering cavities, which would hinder long-term operation. The OFC is centered at a wavelength of 1 pm, which makes it suitable for the investigation of M dwarf stars and the compact size of the OFC-calibrated spectrograph makes it suitable for use in small to mid-scale observatories. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.

We realize a two-stage, hexagonal pyramid magneto-optical trap (MOT) with strontium, and demonstrate loading of cold atoms into cavity-enhanced 1D and 2D optical lattice traps, all within a single compact assembly of in-vacuum optics. We show that the device is suitable for high-performance quantum technologies, focusing especially on its intended application as a strontium optical lattice clock. We prepare 2 × 10 4 spin-polarized atoms of 87 Sr in the optical lattice within 500 ms; we observe a vacuum-limited lifetime of atoms in the lattice of 27 s; and we measure a background DC electric field of 12 V m −1 from stray charges, corresponding to a fractional frequency shift of (−1.2 ± 0.8) × 10 −18 to the strontium clock transition. When used in combination with careful management of the blackbody radiation environment, the device shows potential as a platform for realizing a compact, robust, transportable optical lattice clock with systematic uncertainty at the 10 −18 level.

Details of the NPL Sr optical lattice clock activity are presented, including a full systematic evaluation of NPL's first lattice clock, Sr1, and progress of a second system, Sr2. We will focus discussion of the systematic evaluation on novel methods, such as a Rydberg spectroscopy scheme for determination of the DC Stark shift.

We describe a CW laser stabilized to a low thermal expansion ceramic cavity which has a lower frequency drift rate than cavities based on ultralow-expansion glass (ULE), which are widely used as optical references. Two identical optical cavities with spacers of different material, ceramic and ULE, were assembled and the optical frequencies locked to each of these cavities were compared. The optical frequency drifts of both CW lasers were measured to within a precision of 10-11 in one second over the course of one year. The ceramic cavity had a long-term frequency drift rate of 4.9 mHz/s and the ULE cavity had one of 23 mHz/s.

We describe a CW laser stabilized to a low thermal expansion ceramic cavity which has a lower frequency drift rate than cavities based on ultralow-expansion glass (ULE), which are widely used as optical references. Two identical optical cavities with spacers of different material, ceramic and ULE, were assembled and the optical frequencies locked to each of these cavities were compared. The optical frequency drifts of both CW lasers were measured to within a precision of 10(-11) in one second over the course of one year. The ceramic cavity had a long-term frequency drift rate of 4.9 mHz/s and the ULE cavity had one of 23 mHz/s. (C) 2017 Optical Society of America

We developed a CW laser stabilized to a low expansion ceramic cavity with a frequency drift 7 mHz/s (δf/f ∼ 10 −10 /year), which is lower than those achieved with ultra-low expansion glass cavities. OCIS codes: (140.4780) Optical resonators; (140.3425) Laser stabilization; (120.2230) Fabry-Perot;

Difference-frequency mixing two portions of a broadened Yb: fiber laser spectrum leads to an offset-free supercontinuum. To demonstrate full phase stabilization of the comb, a single comb mode was locked to an optical reference.

A 4-GHz optical frequency comb with a laser-diode pumped Yb:Y2O3 ceramic oscillator is demonstrated. Each comb tooth was resolved by a home-made, sub-gigahertz frequency resolution grating spectrograph, which would be useful for many small observatories.

We present a stand-alone beam focusing flat lens for use in the mid-infrared wavelength range. An aperture and second-order grating structure in a thin gold layer is used to excite and then scatter a propagating surface plasmon polariton (SPP) to constructively interfere in the far-field to produce a narrow beam. A rapidly tunable optical parametric oscillator source is used to demonstrate how changing the wavelength of the incident light from 3.8 - 4.2 μm produces two interfering beams in the far-field caused by the scattering of the propagating SPP interfering with the incident light diffracted by the narrow (sub λ/2) aperture. We identify measured farfield patterns for varying wavelengths using the razor blade edge beam profiling method. The agreement between Finite-Difference Time-Domain (FDTD) modelled and measured results will allow the aperture/grating structures to be integrated directly onto the facets of edge-emitting lasers to dramatically reduce their beam divergence. As edge-emitting lasers have a fixed wavelength, the addition of a thin layer of material such as Silicon or Barium Strontium Titanate to the facets allows tuning by altering the SPP wavelength.

In this work we describe a wavelength-swept continuous-wave optical parametric oscillator (OPO) for the rapid acquisition of mid-infrared spectra spanning over hundreds of wavenumbers. Rapid tuning of a ytterbium-doped fibre pump laser resulted in the OPO idler tuning over 900 cm−1 in 3.36 ms at a resolution of 4.5 cm−1, within a total accessible range of 2.67 to 4.34 μm (2304–3752 cm−1). Predictable tuning characteristics allowed simple online calibration of recorded spectra for absolute mid-infrared frequency. The system thus offers a viable approach to broadband spectral acquisition in applications requiring high-radiance illumination.

A frequency-swept source for broadband mid-infrared spectroscopy is reported. A mid-IR laser source capable of rapidly tuning over up to 900cm -1 within a total accessible range of 2304-3752cm -1 . The rapid tuning range is believed to be the widest reported for an OPO of this type, while the tuning rate and resolution are sufficient for real time IR spectroscopy of solid and liquid samples in techniques requiring high-brightness and wide spectral coverage. The performance and limitations of the OPO system will be discussed and further spectroscopic examples presented.

A continuous-wave all-polarization maintaining ytterbium-doped fiber master oscillator power amplifier, with a tuning range of 70 nm addressable at tuning rates of up to 20 nm/ms, is described. Up to 10 W of linearly polarized output was generated with an amplified spontaneous emission content of less than 0.2% throughout the tuning range.

A Ytterbium-fiber-pumped continuous wave optical parametric oscillator rapidly tunable in the 2.73-4.02µm region is described. The system is well-suited to applications requiring a high-brightness source for spectroscopy of solid and liquid samples.

The first experiments on trapped rotaxanes are presented, combining collision induced fragmentation and in-trap laser spectroscopy. The intrinsic optical properties of three rotaxanes and their non-interlocked building blocks (thread and macrocycle) isolated in a quadrupolar ion trap are investigated. The excitation and relaxation processes under thermal activation as well as under photo-activation are addressed. The light and collision induced fragmentation pathways show that the degradation mechanisms occurring in the rotaxane are highly dependent on the nature of the thread. In the prospective of operating photoswitchable molecules, photo-activation is achieved in a controlled way by depositing photo-energy in the desired sub-unit of a mechanically interlocked structure.