Open-loop polarization mode dispersion mitigation for fibre-optic time and frequency transfer.

The non-reciprocal and dynamic nature of polarization mode dispersion (PMD) in optical fibers can be a problem for accurate time and frequency transfer. Here, a simple, passive solution is put forward that is based on transmitting optical pulses with alternating orthogonal polarization. The fast and deterministic polarization modulation means that the PMD noise is pushed far away from the frequencies of interest. Furthermore, upon reflection from a Faraday mirror at the receiver, the pulses have a well-defined polarization when they return to the transmitter, which facilitates stable optical phase detection and fibre phase compensation. In an open-loop test setup that uses a mode-locked laser and a simple pulse interleaver, the polarization mode dispersion is shown to be reduced by more than two orders of magnitude.

Real-time optical time interpolation using spectral interferometry.

A simple scheme for all-optical time interpolation using spectral interferometry is put forward that is, in principle, capable of single-shot measurements. In this method, the arrival time of optical timing pulses is encoded into the spectrum of a time-stretched supercontinuum via cross phase modulation. The proof-of-concept test setup points toward femtosecond-level absolute timing capabilities with only minor additions to modern optical clockwork.

Spectroscopic thermometry for long-distance surveying.

Electronic distance meters are routinely used to accurately determine the distance between two points. To reach relative measurement uncertainties of 10-7, the average temperature along the beam has to be known within 100 mK since it is a key component in determining the refractive index of air. Temperature measurements at this level are extremely challenging over long distances and especially in an outdoor environment. This paper presents a thermometer for accurate temperature measurements over distances up to a few km. The thermometer is based on direct laser absorption spectroscopy of oxygen near 770 nm. The thermometer yields a spatially continuous measurement of air temperature, and it can provide spatially and temporally well-matching data with an actual distance-measuring laser beam. A field measurement campaign at the 864-m Nummela standard baseline demonstrates applicability of the developed thermometer for improving the refractive index compensation of current high-performance electronic distance meters.

Frequency-comb-referenced mid-infrared source for high-precision spectroscopy.

We report on a tunable continuous-wave mid-infrared optical parametric oscillator (OPO), which is locked to a fully stabilized near-infrared optical frequency comb using a frequency doubling scheme. The OPO is used for 40 GHz mode-hop-free, frequency-comb-locked scans in the wavelength region between 2.7 and 3.4 µm. We demonstrate the applicability of the method to high-precision cavity-ring-down spectroscopy of nitrous oxide (N2O) and water (H2O) at 2.85 µm and of methane (CH4) at 3.2 µm.

Frequency-comb-referenced tunable diode laser spectroscopy and laser stabilization applied to laser cooling.

Laser cooling of trapped atoms and ions in optical clocks demands stable light sources with precisely known absolute frequencies. Since a frequency comb is a vital part of any optical clock, the comb lines can be used for stabilizing tunable, user-friendly diode lasers. Here, a light source for laser cooling of trapped strontium ions is described. The megahertz-level stability and absolute frequency required are realized by stabilizing a distributed-feedback semiconductor laser to a frequency comb. Simple electronics is used to lock and scan the laser across the comb lines, and comb mode number ambiguities are resolved by using a separate, saturated absorption cell that exhibits easily distinguishable hyperfine absorption lines with known frequencies. Due to the simplicity, speed, and wide tuning range it offers, the employed technique could find wider use in precision spectroscopy.

Out-of-Band Fiber-Optic Time and Frequency Transfer Using Asymmetric and Symmetric Opto-Electronic Repeaters.

Signal repeaters for fiber-optic communication can be realized with back-to-back connected transceivers. This configuration can provide high gain ( ≈ 30 dB) at low cost, and the needed semiconductor lasers and modulators can be realized for practically any relevant wavelength. Unfortunately, for time and frequency (TF) transfer the uncorrelated wavelength drifts in the transceiver lasers can compromise transfer stability, and device replacement due to failure may result in large time offsets that have to be measured via global navigation satellite services (GNSS). This work demonstrates that good results can nevertheless be obtained with standard telecom dense wavelength-division multiplexing (DWDM) transceivers over long time periods (years). More importantly, a simple wavelength-symmetric repeater is proposed that can be used to cancel the detrimental effects of wavelength drifts and which lessens the need for link recalibrations after transceiver replacements. In a proof-of-concept test setup, timing drift due to wavelength drift of a repeater laser is reduced by approximately two orders of magnitude.

Simultaneous compression and characterization of ultrashort laser pulses using chirped mirrors and glass wedges.

We present a simple and robust technique to retrieve the phase of ultrashort laser pulses, based on a chirped mirror and glass wedges compressor. It uses the compression system itself as a diagnostic tool, thereby making unnecessary the use of complementary diagnostic tools. We used this technique to compress and characterize 7.1 fs laser pulses from an ultrafast laser oscillator.

Characterization of broadband few-cycle laser pulses with the d-scan technique.

We present an analysis and demonstration of few-cycle ultrashort laser pulse characterization using second-harmonic dispersion scans and numerical phase retrieval algorithms. The sensitivity and robustness of this technique with respect to noise, measurement bandwidth and complexity of the measured pulses is discussed through numerical examples and experimental results. Using this technique, we successfully demonstrate the characterization of few-cycle pulses with complex and structured spectra generated from a broadband ultrafast laser oscillator and a high-energy hollow fiber compressor.

White Rabbit Precision Time Protocol on Long-Distance Fiber Links.

The application of White Rabbit precision time protocol (WR-PTP) in long-distance optical fiber links has been investigated. WR-PTP is an implementation of PTP in synchronous Ethernet optical fiber networks, originally intended for synchronization of equipment within a range of 10 km. This paper discusses the results and limitations of two implementations of WR-PTP in the existing communication fiber networks. A 950-km WR-PTP link was realized using unidirectional paths in a fiber pair between Espoo and Kajaani, Finland. The time transfer on this link was compared (after initial calibration) against a clock comparison by GPS precise point positioning (PPP). The agreement between the two methods remained within [Formula: see text] over three months of measurements. Another WR-PTP implementation was realized between Delft and Amsterdam, the Netherlands, by cascading two links of 137 km each. In this case, the WR links were realized as bidirectional paths in single fibers. The measured time offset between the starting and end points of the link was within 5 ns with an uncertainty of 8 ns, mainly due to the estimated delay asymmetry caused by chromatic dispersion.

Multi-purpose two- and three-dimensional momentum imaging of charged particles for attosecond experiments at 1 kHz repetition rate.

We report on the versatile design and operation of a two-sided spectrometer for the imaging of charged-particle momenta in two dimensions (2D) and three dimensions (3D). The benefits of 3D detection are to discern particles of different mass and to study correlations between fragments from multi-ionization processes, while 2D detectors are more efficient for single-ionization applications. Combining these detector types in one instrument allows us to detect positive and negative particles simultaneously and to reduce acquisition times by using the 2D detector at a higher ionization rate when the third dimension is not required. The combined access to electronic and nuclear dynamics available when both sides are used together is important for studying photoreactions in samples of increasing complexity. The possibilities and limitations of 3D momentum imaging of electrons or ions in the same spectrometer geometry are investigated analytically and three different modes of operation demonstrated experimentally, with infrared or extreme ultraviolet light and an atomic/molecular beam.

Dynamics, bifurcations and chaos in coupled lasers.

Experiments and numerical modelling on two different class B lasers that are subjected to external optical light injection are presented. This presentation includes ways of measuring the changes in the laser output, how to numerically describe the systems and how to construct diagrams of the dynamical states in the plane frequency detuning between lasers and injection strength. The scenarios for the semiconductor laser include an area of frequency locking and islands of chaotic behaviour embedded in and mixed with periodic doubling regimes. Using a rate equation model, the largest Lyapunov exponent is calculated as a measure of the stability of equilibriums and the amount of chaos in chaotic regimes. In the solid-state laser case, different dynamical regions were clearly observed. The found boundaries were identified experimentally, using an identification method, and numerically, from bifurcation analysis, as Hopf, saddle-node, period-doubling and torus bifurcations.

Modulation and the linewidth enhancement factor of a diode-pumped Nd:YVO4 laser.

A technique based on an FM/AM method is proposed for measuring the linewidth enhancement factor a of a diode-pumped Nd:YVO4 laser. A standard rate equation model is complemented with temperature effects to explain the observed behavior under pump modulation. The result alpha = 0.25 +/- 0.13 agrees with values deduced from recent optical injection experiments.