Schmidt-hammer exposure ages from periglacial patterned ground (sorted circles) in Jotunheimen, Norway, and their interpretative problems

© 2016 Swedish Society for Anthropology and Geography Periglacial patterned ground (sorted circles and polygons) along an altitudinal profile at Juvflya in central Jotunheimen, southern Norway, is investigated using Schmidt-hammer exposure-age dating (SHD). The patterned ground surfaces exhibit R-value distributions with platycurtic modes, broad plateaus, narrow tails, and a negative skew. Sample sites located between 1500 and 1925 m a.s.l. indicate a distinct altitudinal gradient of increasing mean R-values towards higher altitudes interpreted as a chronological function. An established regional SHD calibration curve for Jotunheimen yielded mean boulder exposure ages in the range 6910 ± 510 to 8240 ± 495 years ago. These SHD ages are indicative of the timing of patterned ground formation, representing minimum ages for active boulder upfreezing and maximum ages for the stabilization of boulders in the encircling gutters. Despite uncertainties associated with the calibration curve and the age distribution of the boulders, the early-Holocene age of the patterned ground surfaces, the apparent cessation of major activity during the Holocene Thermal Maximum (HTM) and continuing lack of late-Holocene activity clarify existing understanding of the process dynamics and palaeoclimatic significance of large-scale sorted patterned ground as an indicator of a permafrost environment. The interpretation of SHD ages from patterned ground surfaces remains challenging, however, owing to their diachronous nature, the potential for a complex history of formation, and the influence of local, non-climatic factors

Cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy combines broad spectral bandwidth, high spectral resolution, precise frequency calibration, and ultrahigh detection sensitivity, all in one experimental platform based on an optical frequency comb interacting with a high-finesse optical cavity. Precise control of the optical frequency comb allows highly efficient, coherent coupling of individual comb components with corresponding resonant modes of the high-finesse cavity. The long cavity lifetime dramatically enhances the effective interaction between the light field and intracavity matter, increasing the sensitivity for measurement of optical losses by a factor that is on the order of the cavity finesse. The use of low-dispersion mirrors permits almost the entire spectral bandwidth of the frequency comb to be employed for detection, covering a range of ~10% of the actual optical frequency. The light transmitted from the cavity is spectrally resolved to provide a multitude of detection channels with spectral resolutions ranging from a several gigahertz to hundreds of kilohertz. In this review we will discuss the principle of cavity-enhanced direct frequency comb spectroscopy and the various implementations of such systems. In particular, we discuss several types of UV, optical, and IR frequency comb sources and optical cavity designs that can be used for specific spectroscopic applications. We present several cavity-comb coupling methods to take advantage of the broad spectral bandwidth and narrow spectral components of a frequency comb. Finally, we present a series of experimental measurements on trace gas detections, human breath analysis, and characterization of cold molecular beams.Comment: 36 pages, 27 figure

Fundamental Amplitude Noise Limitations to Supercontinuum Spectra Generated in a Microstructured Fiber

Broadband supercontinuum spectra are generated in a microstructured fiber using femtosecond laser pulses. Noise properties of these spectra are studied through experiments and numerical simulations based on a generalized stochastic nonlinear Schrödinger equation. In particular, the relative intensity noise as a function of wavelength across the supercontinuum is measured over a wide range of input pulse parameters, and experimental results and simulations are shown to be in good quantitative agreement. For certain input pulse parameters, amplitude fluctuations as large as 50% are observed. The simulations clarify that the intensity noise on the supercontinuum arises from the amplification of two noise inputs during propagation - quantum-limited shot noise on the input pulse, and spontaneous Raman scattering in the fiber. The amplification factor is a sensitive function of the input pulse parameters. Short input pulses are critical for the generation of very broad supercontinua with low noise.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Amplitude noise on supercontinuum generated in microstructure fiber: Measurements and simulations

Supercontinua generated in microstructure fiber can exhibit significant excess amplitude noise. We present experimental and numerical studies of the origins of this excess noise and its dependence on the input laser pulse parameters.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

Portable acetylene frequency references inside sealed hollow-core kagome photonic crystal fiber

A continuous-wave diode laser is stabilized to a near-infrared acetylene transition inside a sealed kagome photonic crystal fiber. Stability and absolute frequency are measured with a frequency comb, and polarization sensitivity is observed

Soliton generation via intrapulse stimulated Raman scattering in photonic crystal fibers: Experimental and numerical investigations

We investigate femtosecond pulse propagation in photonic crystal fiber, reporting the generation of tunable femtosecond soliton pulses. For sufficiently broad spectral content, stimulated Raman scattering transfers energy from the higher frequency spectral components to lower frequencies, resulting in a continuous self-frequency shift to longer wavelengths. Power dependent spectral analysis reveals a well-formed soliton at peak powers exceeding 100 W. Background-free intensity autocorrelation measurements confirm soliton formation with a duration of < 90 fs and with an energy conversion efficiency of 60%. Numerical solutions were performed based on a generalized nonlinear Schrödinger equation that included the effects of dispersion, self-steepening, optical shock formation, self-phase modulation and stimulated Raman scattering. The resulting spectra from the simulations are in excellent agreement with the measured spectra, and are consistent with the intensity autocorrelation measurements.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

Femtosecond soliton generation in air-silica microstructure fibers

Femtosecond soliton generation in air-silica microstructure fibers (ASMF) was investigated. The generation of femtosecond Raman soliton pulses tunable over 200 nm in the near infrared was reported. The intensity and phase of the input pulse was determined by second-harmonic generation frequency-resolved optical gating. An experimentally determined Raman response for standard silica fibers was used in the simulations that included both instantaneous electronic and delayed Raman contributions. The spectra resulting from the simulations were in agreement with the measured spectra. The numerical temporal behaviour was also consistent with the experimentally measured intensity autocorrelation.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Erratum: Fundamental amplitude noise limitations to supercontinuum spectra generated in a microstructured fiber (Appl. Phys. B (2003) 77 (269-277))

SCOPUS: er.jinfo:eu-repo/semantics/publishe