Sub-kHz-linewidth VECSELs for cold atom experiments

We report and characterize sub-kHz linewidth operation of an AlGaInP-based VECSEL system suitable for addressing the narrow cooling transition of neutral strontium atoms at 689 nm. When frequency-stabilized to a standard air-spaced Fabry-Perot cavity (finesse 1000) via the Pound-Drever-Hall (PDH) technique, it delivers output power >150 mW in a circularly-symmetric single transverse mode with low frequency and intensity noise. The optical field was reconstructed from the frequency noise error signal via autocorrelation and the Wiener-Khintchine theorem, leading to an estimated linewidth of (125±2) Hz. Optical beat note measurements were performed against a commercial locked laser system and a second, almost identical, VECSEL system resulting in linewidths of 200 Hz and 160 Hz FWHM, respectively. To the best of our knowledge, this is the first demonstration of a VECSEL compatible with the narrowest of lines (few hundred Hz) used for cooling and trapping atoms and ions

Electromagnetically Induced Transparency (EIT) aided cooling of strontium atoms

The presence of ultra-narrow inter-combination spectroscopic lines in alkaline earth elements places them as promising candidates for optical atomic clocks, quantum computation, and for probing fundmental physics. Doppler cooling of these atoms is typically achieved through two subsequent stages: the initial cooling is on the 1s0-1p1 transition followed by cooling using the narrow-line 1s0-3p1 transition. However, due to significantly lower linewidth of the second stage cooling transition, efficient transfer of atoms into the second stage becomes technically challenging. The velocity distribution of the atoms after the first stage of cooling is too broad for atoms to be captured efficiently in the second stage cooling. As a result, the capture efficiency of atoms into the second stage Magneto-Optical Trap is low, even if the linewidth of the second stage cooling laser is artificially broadened.Comment: 7 pages, 3 figure

Novel repumping on 3^{3}P0_{0}→\rightarrow3^{3}D1_{1} for Sr magneto-optical trap and Land\'e g factor measurement of 3^{3}D1_{1}

We realize an experimental facility for cooling and trapping strontium (Sr) atoms and measure the Land\'e g factor of $^{3}$D$_{1}$ of $^{88}$Sr. Thanks to a novel repumping scheme with the $^{3}$P$_{2}$$\rightarrow$$^{3}$S$_{1}$ and $^{3}$P$_{0}$$\rightarrow$$^{3}$D$_{1}$ combination and the permanent magnets based self-assembled Zeeman slower, the peak atom number in the continuously repumped blue MOT is enhanced by a factor of 15 with respect to the non-repumping case, and reaches $\sim$1 billion. Furthermore, using the resolved-sideband Zeeman spectroscopy, the Land\'e g factor of $^{3}$D$_{1}$ is measured to be 0.4995(88) showing a good agreement with the theoretical value of 0.4988. The results will have an impact on various applications including atom laser, dipolar interactions, quantum information and precision measurements.Comment: 8 pages, 9 figure

Microwave Photonics in Networked Staring Radar

Modern radar systems are capable of detecting small moving objects such as drones on the kilometer scale. The complex and evolving environment poses challenges such as interference, clutter induced phase noise and obstruction of targets. Networked radar systems are a potential solution but also bring their own challenges such as synchronization. In this paper, the effect of the oscillator on the networked radar is discussed and how microwave photonics are able to be integrated into the network for superior phase noise and synchronization performance

Whole system radar modelling::Simulation and validation

The ever-expanding horizon of radar applications demands solutions with high-end radar functionalities and technologies and is often limited by the available radar equipment, cost and time. A practical method to tackle the situation is to rely on the modelling and simulation of radar systems based on the user requirements. The comprehensive system-level modelling of a pulsed Doppler radar in MATLAB/Simulink consisting of all the fundamental blocks in the transmit chain, the environment, the receive chain, and the data processing chain is presented in this article. The first half of the article discusses the high-fidelity simulation of each building block in the radar model. In the second half of the article, the range-Doppler plot generated from the high-fidelity radar model is compared and validated using the range-Doppler plot from a real radar trial. The radar phase noise plays a crucial role in the detection of slowly moving, low radar cross-section targets in the presence of strong clutter. The article also briefly discusses the effects of radar oscillator phase noise in the range-Doppler plot. The validated, fully flexible radar model has the advantage of supporting the addition of further building blocks and optimising the parameters based on user requirements

Development of a networked photonic‐enabled staring radar testbed for urban surveillance

Urban surveillance of slow-moving small targets such as drones and birds in low to medium airspace using radar presents significant challenges. Detecting, locating and identifying such low observable targets in strong clutter requires both innovation in radar hardware design and optimisation of processing algorithms. To this end, the University of Birmingham (UoB) has set-up a testbed of two L-band staring radars to support performance benchmarking using datasets of target and clutter from realistic urban environment. This testbed is also providing the vehicle to understand how novel radar architectures can enhance radar capabilities. Some of the challenges in installing the radar at the UoB campus are highlighted. Detailed benchmarking results are provided from urban monostatic and bistatic field trials that form the basis for performance comparison against future hardware modification. The solution to the challenge of interfacing the radar to the external oscillators is described and stand-alone bench tests with the candidate oscillators are reported. The testbed provides a valuable capability to undertake detailed analysis of performance of Quantum photonic-enabled radar and allows for its comparison with conventional oscillator technology for surveillance of low observable targets in the presence of urban clutter

A high-performance optical lattice clock based on bosonic atoms

Optical lattice clocks with uncertainty and instability in the $10^{-17}$-range and below have so far been demonstrated exclusively using fermions. Here, we demonstrate a bosonic optical lattice clock with $3\times 10^{-18}$ instability and $2.0\times 10^{-17}$ accuracy, both values improving on previous work by a factor 30. This was enabled by probing the clock transition with an ultra-long interrogation time of 4 s, using the long coherence time provided by a cryogenic silicon resonator, by careful stabilization of relevant operating parameters, and by operating at low atom density. This work demonstrates that bosonic clocks, in combination with highly coherent interrogation lasers, are suitable for high-accuracy applications with particular requirements, such as high reliability, transportability, operation in space, or suitability for particular fundamental physics topics. As an example, we determine the $^{88}\textrm{Sr} - ^{87}$Sr isotope shift with 12 mHz uncertainty

Ultra-stable clock laser system development towards space applications

International audienceThe increasing performance of optical lattice clocks has made them attractive for scientific applications in space and thus has pushed the development of their components including the interrogation lasers of the clock transitions towards being suitable for space, which amongst others requires making them more power efficient, radiation hardened, smaller, lighter as well as more mechanically stable. Here we present the development towards a space-compatible interrogation laser system for a strontium lattice clock constructed within the Space Optical Clock (SOC2) project where we have concentrated on mechanical rigidity and size. The laser reaches a fractional frequency instability of 7.9 × 10−16 at 300 ms averaging time. The laser system uses a single extended cavity diode laser that gives enough power for interrogating the atoms, frequency comparison by a frequency comb and diagnostics. It includes fibre link stabilisation to the atomic package and to the comb. The optics module containing the laser has dimensions 60 × 45 × 8 cm3; and the ultra-stable reference cavity used for frequency stabilisation with its vacuum system takes 30 × 30 × 30 cm3. The acceleration sensitivities in three orthogonal directions of the cavity are 3.6 × 10−10/g, 5.8 × 10−10/g and 3.1 × 10−10/g, where g ≈ 9.8 m/s2 is the standard gravitational acceleration