Polarisation compensation in non-planar image-rotating OPO ring resonators.

Non-planar image-rotating OPO ring resonators necessitate polarisation compensation in contrast to their planar counterparts. This is essential for maintaining phase matching conditions for non-linear optical conversion in the resonator during each cavity round trip. In this study, we examine the polarisation compensation and its impact on the performance of two types of non-planar resonators: RISTRA with a π2 image rotation and FIRE with a fractional image rotation (π2 fraction). The RISTRA is insensitive to mirror phase shifts, while the FIRE has a more complex dependence of polarisation rotation on mirror phase shifts. There has been debate over whether a single birefringent element can provide adequate polarisation compensation for non-planar resonators beyond RISTRA-type. Our results show that under certain experimentally feasible conditions, even FIRE resonators can achieve adequate polarisation compensation with a single half-wave plate. We validate our theoretical analysis through numerical simulations and experimental studies of OPO output beam polarisation using ZnGeP2 non-linear crystals.

Effects of pump pulse energy and repetition rate on beam quality in a high-power mid-infrared ZnGeP2 OPO.

We report on changes of beam quality with rising pump pulse energy and repetition rate in a high-power mid-infrared ZnGeP2 (ZGP) OPO in a non-planar cavity. The ZGP OPO generates 25 W in 3 - 5 µm pumped by a 10 kHz Q-switched Ho:LLF MOPA system at 2 µm. The beam quality factor increases from nearly diffraction-limited at OPO threshold to a M2 = 1.8 at maximum pump power. The degradation can be attributed to thermal lensing and gain guiding effects in the nonlinear crystal. To discriminate between those effects, we monitored the beam quality as a function of the repetition rate at the highest pump pulse energy using a Pockels cell as a pulse picker. Therefore, the heat load inducing thermal lensing increases linearly, whereas gain guiding remains nearly unaffected. M2 shows a minimum of 1.2 at 1 kHz, starting from 1.3 at 100 Hz and rising to 1.8 at 10 kHz. Those findings have been reproduced by numerical simulations. Further investigations show the transient evolution of beam quality, revealing the thermal lens build-up process to the steady-state.

Beam quality and efficiency of ns-pulsed high-power mid-IR ZGP OPOs compared in linear and non-planar ring resonators.

We compare the performance of ns-pulsed mid-infrared optical parametric oscillators (OPOs) based on ZGP crystals in a simple linear cavity and two non-planar image-rotating resonators. Our 2 µm pump source is a 68 W 10 kHz diffraction limited Ho:LLF MOPA system. In a linear cavity, it allows us to generate up to 38 W of the total average power with M2<2.2 from the ZGP OPO in the 3-5 µm range. Beam quality and stability are significantly improved in both types of non-planar ring resonators (M2<1.8 in RISTRA and M2<1.4 in FIRE), while the total average power is above 21 W. Factors influencing the beam quality such as average power and pump repetition rate are studied in detail.

Evaluation of a Fatigue Detector Using Eye Closure-Associated Indicators Acquired from Truck Drivers in a Simulator Study.

This paper presents a camera-based prototype sensor for detecting fatigue and drowsiness in drivers, which are common causes of road accidents. The evaluation of the detector operation involved eight professional truck drivers, who drove the truck simulator twice-i.e., when they were rested and drowsy. The Fatigue Symptoms Scales (FSS) questionnaire was used to assess subjectively perceived levels of fatigue, whereas the percentage of eye closure time (PERCLOS), eye closure duration (ECD), and frequency of eye closure (FEC) were selected as eye closure-associated fatigue indicators, determined from the images of drivers' faces captured by the sensor. Three alternative models for subjective fatigue were used to analyse the relationship between the raw score of the FSS questionnaire, and the eye closure-associated indicators were estimated. The results revealed that, in relation to the subjective assessment of fatigue, PERCLOS is a significant predictor of the changes observed in individual subjects during the performance of tasks, while ECD reflects the individual differences in subjective fatigue occurred both between drivers and in individual drivers between the 'rested' and 'drowsy' experimental conditions well. No relationship between the FEC index and the FSS state scale was found.

Dual Hg-Rb magneto-optical trap.

We present a two-species laser cooling apparatus capable of simultaneously collecting Rb and Hg atomic gases into a magneto-optical trap (MOT). The atomic sources, laser system, and vacuum set-up are described. While there is a loss of Rb atoms in the MOT due to photoionization by the Hg cooling laser, we show that it does not prevent simultaneous trapping of Rb and Hg. We also demonstrate interspecies collision-induced losses in the 87Rb-202Hg system.

A single-atom 3D sub-attonewton force sensor.

Forces drive all physical interactions. High-sensitivity measurement of the effect of forces enables the quantitative investigation of physical phenomena. Laser-cooled trapped atomic ions are a well-controlled quantum system whose low mass, strong Coulomb interaction, and readily detectable fluorescence signal make them a favorable platform for precision metrology. We demonstrate a three-dimensional sub-attonewton sensitivity force sensor based on a super-resolution imaging of a single trapped ion. The force is detected by measuring the ion's displacement in three dimensions with nanometer precision. Observed sensitivities were 372 ± 9, 347 ± 18, and 808 ± 51 zN/[Formula: see text], corresponding to 24×, 87×, and 21× above the quantum limit. We verified this technique by measuring a 95-zN light pressure force, an important systematic effect in optically based sensors.

Preliminary Results of the LF/HF Ratio as an Indicator for Estimating Difficulty Level of Flight Tasks.

INTRODUCTION: The main aim of this study was to differentiate the magnitude of a pilot's heart rate variability (HRV) when performing assisted and unassisted flights, as well as simple and complex flight tasks. METHODS: Cardiac monitoring in flights was carried out using a compact, mobile ECG recorder. A frequency analysis of the heart rate (HR) signal was performed to determine the ratio of low-frequency spectral power (LF) to high-frequency spectral power (HF). RESULTS: The LF/HF ratio observed in the zone (M=1.047, SD=0.059) was significantly different than the LF/HF calculated preflight (M=0.877, SD=0.043) and postflight (M=0.793, SD=0.037). There was no main effect of the flight type (unassisted zone flight vs. zone flight with an instructor) on the LF/HF parameter. However, greater psychophysiological load of a pilot was observed in the training zone flights when compared to simple circle flights (main effect of the flight type). CONCLUSIONS: As the LF/HF ratio turned out to be significantly higher in the zone than pre- and postflight, this parameter can be useful for predicting the risk of excessive stress and arousal of pilots during flights. Based on the LF/HF ratio we can also estimate difficulty level of flight tasks, because our research has shown higher values of this parameter in the training zone flights than in simple circle flights.