High-sensitivity dual-comb and cross-comb spectroscopy across the infrared using a widely-tunable and free-running optical parametric oscillator

Coherent dual-comb spectroscopy (DCS) is a form of Fourier transform spectroscopy that enables high-resolution measurements at high speeds without the trade-off between resolution and update rate inherent to mechanical delay scanning. However, high complexity of the optical system and limited sensitivity of the measurements remain major challenges for deploying broadband DCS in the short-wave infrared (SWIR, 1.4-3 um) and mid-infrared (mid-IR, >3 um) regions where many molecules have strong absorption bands. We address these challenges via a wavelength-tunable dual-comb optical parametric oscillator (OPO) combined with a new detection method. Both OPO pump beams are generated in a single spatially-multiplexed laser cavity, while both signal and idler beams are generated in a single spatially-multiplexed OPO cavity. The near-common-path of the beams in each cavity ensures that even in free-running operation the noise sources of the two combs are highly correlated, facilitating comb-line-resolved and aliasing-free measurements with 250-MHz spectral resolution. At an instantaneous bandwidth below 1 THz, high power per comb line of up to 70 uW (signal) and 150 uW (idler) is achieved. The accessible spectrum spans 1290 nm to 1670 nm (signal) and 2700 nm to 5160 nm (idler). In a SWIR DCS measurement, we achieve a signal-to-noise ratio (SNR) of 33 dB Hz^1/2. For mid-IR measurements, we introduce a novel cross-comb spectroscopy implementation to overcome limitations posed by traditional mid-IR detection, obtaining a record-high SNR of 41 dB Hz^1/2. Our results are a promising route towards dual-comb spectroscopy with high sensitivity and high resolution over a wide spectral range

Picosecond ultrasonics with a free-running dual-comb laser

We present a free-running 80-MHz dual-comb polarization-multiplexed solid-state laser which delivers 1.8 W of average power with 110-fs pulse duration per comb. With a high-sensitivity pump-probe setup, we apply this free-running dual-comb laser to picosecond ultrasonic measurements. The ultrasonic signatures in a semiconductor multi-quantum-well structure originating from the quantum wells and superlattice regions are revealed and discussed. We further demonstrate ultrasonic measurements on a thin-film metalized sample and compare these measurements to ones obtained with a pair of locked femtosecond lasers. Our data show that a free-running dual-comb laser is well-suited for picosecond ultrasonic measurements and thus it offers a significant reduction in complexity and cost for this widely adopted non-destructive testing techniqu

Conditional variable importance for random forests

Random forests are becoming increasingly popular in many scientific fields because they can cope with ``small n large p'' problems, complex interactions and even highly correlated predictor variables. Their variable importance measures have recently been suggested as screening tools for, e.g., gene expression studies. However, these variable importance measures show a bias towards correlated predictor variables. We identify two mechanisms responsible for this finding: (i) A preference for the selection of correlated predictors in the tree building process and (ii) an additional advantage for correlated predictor variables induced by the unconditional permutation scheme that is employed in the computation of the variable importance measure. Based on these considerations we develop a new, conditional permutation scheme for the computation of the variable importance measure. The resulting conditional variable importance is shown to reflect the true impact of each predictor variable more reliably than the original marginal approach

51-W average power, 169-fs pulses from an ultrafast non-collinear optical parametric oscillator

ISSN:1094-408

Dual-comb optical parametric oscillator in the mid-infrared based on a single free-running cavity

We demonstrate a free-running single-cavity dual-comb optical parametric oscillator (OPO) pumped by a single-cavity dual-comb solid-state laser. The OPO ring cavity contains a single periodically-poled MgO-doped LiNbO3 (PPLN) crystal. Each idler beam has more than 245-mW average power at 3550 nm and 3579 nm center wavelengths (bandwidth 130 nm). The signal beams are simultaneously outcoupled with more than 220 mW per beam at 1499 nm and 1496 nm center wavelength. The nominal repetition rate is 80 MHz, while the repetition rate difference is tunable and set to 34 Hz. To evaluate the feasibility of using this type of source for dual-comb applications, we characterize the noise and coherence properties of the OPO signal beams. We find ultra-low relative intensity noise (RIN) below -158 dBc/Hz at offset frequencies above 1 MHz. A heterodyne beat note measurement with a continuous wave (cw) laser is performed to determine the linewidth of a radio-frequency (RF) comb line. We find a full-width half-maximum (FWHM) linewidth of around 400 Hz. Moreover, the interferometric measurement between the two signal beams reveals a surprising property: the center of the corresponding RF spectrum is always near zero frequency, even when tuning the pump repetition rate difference or the OPO cavity length. We explain this effect theoretically and discuss its implications for generating stable low-noise idler combs suitable for high-sensitivity mid-infrared dual-comb spectroscopy (DCS).ISSN:1094-408

Association analysis of frost tolerance in rye using candidate genes and phenotypic data from controlled, semi-controlled, and field phenotyping platforms

<p>Abstract</p> <p>Background</p> <p>Frost is an important abiotic stress that limits cereal production in the temperate zone. As the most frost tolerant small grain cereal, rye (<it>Secale cereale </it>L.) is an ideal cereal model for investigating the genetic basis of frost tolerance (FT), a complex trait with polygenic inheritance. Using 201 genotypes from five Eastern and Middle European winter rye populations, this study reports a multi-platform candidate gene-based association analysis in rye using 161 single nucleotide polymorphisms (SNPs) and nine insertion-deletion (Indel) polymorphisms previously identified from twelve candidate genes with a putative role in the frost responsive network.</p> <p>Results</p> <p>Phenotypic data analyses of FT in three different phenotyping platforms, controlled, semi-controlled and field, revealed significant genetic variations in the plant material under study. Statistically significant (<it>P </it>< 0.05) associations between FT and SNPs/haplotypes of candidate genes were identified. Two SNPs in <it>ScCbf15 </it>and one in <it>ScCbf12</it>, all leading to amino acid exchanges, were significantly associated with FT over all three phenotyping platforms. Distribution of SNP effect sizes expressed as percentage of the genetic variance explained by individual SNPs was highly skewed towards zero with a few SNPs obtaining large effects. Two-way epistasis was found between 14 pairs of candidate genes. Relatively low to medium empirical correlations of SNP-FT associations were observed across the three platforms underlining the need for multi-level experimentation for dissecting complex associations between genotypes and FT in rye.</p> <p>Conclusions</p> <p>Candidate gene based-association studies are a powerful tool for investigating the genetic basis of FT in rye. Results of this study support the findings of bi-parental linkage mapping and expression studies that the <it>Cbf </it>gene family plays an essential role in FT.</p

Intraoperative transit time flow measurement: off-pump versus on-pump coronary artery bypass

Hassanein W, Albert AA, Arnrich B, et al. Intraoperative transit time flow measurement: off-pump versus on-pump coronary artery bypass. The annals of thoracic surgery. 2005;80(6):2155-2161.Background Off-pump coronary artery bypass grafting (OPCAB) has attracted increasing attention. Performing the anastomosis off-pump is technically more demanding. The objective of the study is to assess the quality of anastomosis in OPCAB in comparison with conventional on-pump coronary artery bypass grafting using the transit time flow measurement. Methods Four hundred forty-five patients operated on using OPCAB technique were included in the study. For each patient in this group a similar patient from the on-pump coronary artery bypass grafting population was selected according to the number of grafts, bypass material, and target coronary arteries. The mean flow and the pulsatile index were measured in every bypass graft in both groups. Results The average pulsatile index in OPCAB was 2.09 ± 1.03 (mean flow, 39 ± 22.63 mL/min), whereas with on-pump coronary artery bypass grafting it was 1.9 ± 0.98 (mean flow, 44.19 ± 23.58 mL/min); p = 0.005. Subgroup analysis showed significantly lower mean flows and higher pulsatile index with OPCAB in grafts to the obtuse marginal, diagonal, and right coronary artery, but not to the left anterior descending territory. Conclusions The quality of the anastomosis performed using the OPCAB technique might be jeopardized by less accessibility as in the case of lateral and posterior wall coronary arteries. Techniques to optimize the accessibility of the coronary artery like combining sling support with cup stabilizers, together with systematic training, should be strongly considered in OPCAB. Whenever there is good accessibility of the coronary artery as in the case of the left anterior descending, the anastomosis performed under OPCAB has a quality as good as that performed using the conventional technique